In the whacky-world-of windows,, there is no way to narrow it down to just a modem that quickly.

Are you running any extras (RogerWilco etc.) when on-line ?

Does it crash and lock up,, or crash and reboot  ?

If it locks up,,at random, I had a similar prob and it turned out to be a over-heating graphics card (I know,,wierd that it only happened online, maybe it was having to calculate and display all the other planes,, but a fan solved it) (over-crowded slots    )

If it reboots,, I had that prob also, but I can't mention the solution here,, people yell at me  

I'm not very up on P.C stuff at all. I've been using them for years and am quite proficient in operating problems and have a general (very general) understanding of the basic functions etc.
But since beginning Simming 18 mths ago, I have simply found myself out of my element. I am above average intelligence and am not afraid of new technology (although electronics have always been a mystery to me, I'm one of those kids that DIDN"T build a chrystal set............lol).
Obviously, the answer is to take a little of the spare hours I have and do some kind of computer course at College. Maybe I will.

The crux of all this, however, is that it's not you, or even the technology, I think it's the Simming habit we seem to have developed. Especially M$ Sims............lol

Have you ever had these sorts of problems with M$ Word or Excel or Encyclopaedia Britannica programs...................I'll bet not.