Apple Mail Account: Only Downloading 1 Email at a Time

From about three weeks ago, on only my most heavily used email account (it has around 75,000 emails in it and gets around 200 emails each day), I was only able to download 4 emails at a time. Then it would stop. I would click "Get Mail" and it would download the next 4 emails. I'd click "Get Mail" etc etc until all emails were downloaded.

I tried to find a solution, but couldn't, so I just put up with it (it was really only a pain in the butt first thing in the morning).

Then, this morning - disaster! My main account only downloads one email at a time! It shows "55 of 55 Incoming messages", then next time "55 of 55 incoming messages" etc. A new one must have arrived because now it is showing "56 out of 56" (but I still have 6 hours of emails to download).

Important info:

* Email is hosted by gmail.
* Account is setup as a POP account.
* This issue is not affecting other staff on a mac, using same domain for email. Nor is it affecting any other email accounts I have (about 16 active email accounts, including 2 others hosted by gmail).
* Mail version is 4.3
* OS is 10.6.4
* Hard drive has 83GB available
* Last week I installed an extra 2GB of ram - so it wont be a lack of ram.
* No other email accounts are affected.
* There are 20 unread emails sitting in my Gmail account waiting to be downloaded (I logged in to Google Webmail and saw them).
* I have checked the Activity Monitor for Mail and that account seems to act absolutely perfectly (if it only had 1 email to download). In other words, there is nothing in the Activity Monitor that suggests a problem.
Any suggestions would be much appreciated!

MBP 2.4GHZ, 6/320 Mac OS X (10.6.4)