Uploading to online data storage: Now cannot download songs into iTunes

Question

Hello all.
I am not seeing this one on any FAQs, knowledge bases, or discussion boards yet:
I am doing my initial upload of files to SugarSync, a highly-recommended online data storage service. Since I started, I cannot download songs into iTunes, even from the iTunes Store.
On any new downloads from the iTunes Store, the song DOES appear in the Music library view but immediately after completing the download it shows the exclamation point warning that the file is not in its location. The new folders get created properly within the iTunes Music folder but the subfolder where the song should be is empty. Each time I had to write iTunes Support to get them to make the files available.
I can still add files manually, no problem. E.g., I can add *.mp3 or *.wav files or folders, I can convert them to AAC, etc. The glitch seems to occur only when automated loads into iTunes are operating.
As a test, I downloaded a song from Amazon. Here the problem was different but I could work around it manually. Normally the Amazon process loads songs automatically into iTunes, too. Here again, the download did create the proper folder in the iTunes Music folder, as it should, but this time the symptoms were reversed:
(a) the mp3 file WAS in its folder as it should be (w/the iTunes DL, the file was NOT there)
(b) the song did NOT appear in the iTunes Music view (w/the iTunes DL, the song DID appear)
(c) I was able to browse to the file and tell it manually to load into iTunes (w/iTunes I had to write Support and wait a day).
(I wonder what's the cause of the differences between the two cases.)
Strictly speaking, I can't PROVE the problem has anything to do with SugarSync (which otherwise seems good so far), but the DL problem started as soon as I started using it. Something in the SugarSync upload or file-monitoring process, or an odd thin gin iTunes, seems to be preventing automated, direct loads into iTunes. And since the data service runs in background, so it can monitor file changes, that might mean I can't buy music anymore! Obviously that would be a dealbreaker with SS. (I have contacted SS on this but they've not had fair chance to reply yet.)
1) Anyone else have this problem?
2) Is this permanent or just temporary while I am doing the initial upload?
3) Anyone know a solution?
(FYI, I am a highly-experienced user and otherwise quite handy with iTunes files, library moves, and backups. My library is entirely consolidated and all AAC.)
Thanks.
(Oh, and this occurred in both iTunes 8 and the new iTunes 9, so it seems unrelated to the upgrade this week.)

Answer 1

UPDATE 1. CHANGING BROWSER HELPED -- OR DID IT?
I called Apple iTunes Support, who said the problem is new to them. The technician's hypothesis was that something, perhaps browser-related, was interfering with the initial creation of a temporary file (which should go to the Recycle Bin) that instead causes the completed file to go to there.
He noted that iTunes, though not going through one's browser onscreen, does use settings within one's default browser. I use Mozilla Firefox, so we switched to IE as the default browser, restarted iTunes, and the song downloaded with no problem! Then I switched back to Mozilla, restarted iTunes, and it worked AGAIN with no problem!
(Dutifully I advised SugarSync, which is still investigating.)
UPDATE 2: ARTIST NAME CHANGE - SOME FILES GOT MIS-MOVED / MIS-CHANGED
Definitely something still wrong. This time some pre-existing song entries (not new downloads) lost their connection to their source file.
In iTunes, which manages folder names for artists and albums automatically, I corrected the spelling for an Artist, so immediately iTunes renamed the folder, and automatically SugarSync noted the change to be uploaded. While the changed folder name and all the songs within were still uploading to SS, in iTunes I saw exclamation points come up -- but only for some of them. Most files got moved or changed correctly, but several lost connection to their file (i.e., the file was removed for the original misnamed folder but never moved into the correctly-named folder). Weird.
Worse, in only some of those cases did I find the missing *.m4a file in the Recycle bin. (I had to retrieve old, original *.mp3 versions from another folder and re-import each into iTunes manually.) I've never seen iTunes have a problem managing an Artist rename until I started using the live SS process.
(I've reported this to SS and asked if there is a way to disable temporarily SS to see if that's the problem.)
[Note: I am willing to try downloads again but I am wary of trying to rename entire Artists (Folder) again. That was a lot of work.]
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UPDATE 3: SERIES OF TESTS - 1 FAILURE USING iTUNES
Still problem occurs, but not always. Today, I rebooted PC. I tried CD, iTunes, & Amazon. I varied having the browser open when using iTunes.
Here are the results of a series of attempts to download songs. "FAIL" means the file did not load properly into iTunes or loaded but lost its connection (exclamation point warning).
# Source Mozilla #Songs Result
1 CD Closed 1 OK
2 iTunes Closed 1 OK
3 Amazon Open 2 OK
4 iTunes Open 1 FAIL
5 Amazon Open 1 OK
6 iTunes Closed 1 OK
7 Amazon Open 2 OK
8 iTunes Open 2 OK
(I reported this to SS. Hoping they'll test and find the problem.)