Gradual OUT OF SYNC while using TMPGEen

[Makeitso]

I have been creating my own VCD's for a little while now using my TV Card and Ulead Multimedia Studio & DVD Factory.
However, As soon as I started using TMPGEnc and VirtualDub, I was hooked immediately on their ease of use and flexibility.
The only problem I really have (and keep on getting more and more recently), is the gradual audio out-of-sync on some (NOT ALL) movies/clips that I download.
I have read the Forums (as much as humanly possible, considering the thousands of posts), but time and time again I read solutions to resolving the constant out-of-sync problem, which is relatively easy to do when following the instructions on the main site.
I have noticed that when this problem occurs the movie always starts out fine, but by the end of the movie, it could get as much as 30 seconds or so out-of sync.
I tried using my video editor to check the actual lenght of the wav file created by VIRTUALDUB, in comparison with the actual lenght of the original movie, but they are always the same length...Just out-of-sync!
I really am not interested in going through any major lenthy procedures to correct these problems, but I would really like to know what is causing them in the first place so that I can make adjustments or alterations for the future. I have noticed that the problem is most prevalent when converting from DivX movies, but not always!
I try to limit my downloads to MPEG 353x288 now, but as I live in the UK, I still have to do the conversion for the framerate to PAL (from 29. to 25 FPS), and then sometimes, not always, I get the out of sync error again. I read somewhere that it could have something to do with the bitrate of the movie, but I'm not sure what that means, or what to do about it.
Is this 'out-of-sync' problem limited to the use of TMPGEnc & VirtualDub, or has anyone else, using other programs had the same probs?
One final note...I have managed to back up some of my own DVD's in the past, using the above programs and others from start to finish to VCD, with no problem, so I must assume it's something wrong with the way that some people are making these movies in the first place.....?
Thanx in advance for any help.

[Makeitso]

OK... Does the fact that no one has replied to my post mean that no one knows the answer, or is it that the answer is out there somewhere amonst the thousands of posts, and no one can be bothered to tell me again?
I have honestly looked again and again, but no post I have found actually answers my question fully. I am aware that there are posts relating to out of sync movies, but if you read my question you'll find it's not as simple as that!
PLEASE HELP!!!!!