IMAP. Two Leopard Macs. 1st Mac sees file attachment 2nd Mac doesn't

Question

I use two Mac computers both on 10.5.2, each running Apple Mail 3.2 to access Gmail's IMAP.
Worked well for ages, but recently, in one computer, I sent an email with a very large 3MB file attachment, and then transferred it to a folder. That was then reflected in Gmail's IMAP interface. Hence, as far as Gmail is concerned, the file attachment and email are there.
But when I used the 2nd Mac to download the same email, the email is there, but no file attachment.
When I open the particular email, there's an error message at the top of the email:
Apple-Mail-9-1026228142
Content-Type: text/plan; charset=WINDOWS-1252; format=flowed; delsp=yes
Content- Transfer-Encoding: quoted-printable
I've never seen that before.
Also, in the 2nd Mac, in Apple Mail, there is no paper clip icon that is usually associated with a file attachment.
Even stranger: A few days later, I initially used the 2nd Mac to download an email with a file attachment. So the email and file attachment are on the 2nd Mac. I verify that the email+file attachment are on Gmail's IMAP web interface.
The strange thing, however, is that the 1st Mac now only seems the same email, but no file attachment.
It almost seems as if whichever Mac I first use (either the 1st or 2nd Mac) to download an email with file attachment, that initial Mac sees the attachment. But subsequently, when the other of the Macs downloads the same attachment, it does not have the attachment.
As said, Gmail's webmail IMAP indicates the attachment is there.
Any ideas?

Answer 1

I figured it out. I had been connecting to my router with Ethernet AND Airport (both were turned on in my Network preferences, and both had a green-light status of 'connected'). Don't know why that would have affected it, but when I turned OFF the airport connection everything was instantaneously corrected.