IMAP every 15 minutes not reliable - Am I the only one?

Question

I had the problem since 1.1.2 (on an unhacked T-Mobile Germany iPhone) and it seems to get worse with 1.1.3.
Pulling mails using IMAP from my .mac account works for an hour or some hours then it stops to pull mails every 15 minutes. When I manually start the mail program it loads all mails. So my settings are correct. When I reboot my iPhone the problem is gone for a while but then comes back. Am I the only one because I read no other post with this specific problem.
Deleting and recreating the mailbox on the phone did not help (and I found out that deleting a mailbox from the phone does NOT delete all information from the phone, a cache of the IMAP messages is still kept). Even a full recovery (including restore from backup) on the phone did not help.

Answer 1

I solved the problem now this way:
- Did a complete reset of the iPhone and
- I did NOT restore from a previous backup.
Instead I entered all my settings manual again, like IMAP Servers, all phone settings etc.
For now the problem is gone. Works reliable for a week or so. Knocking on wood...