A few friends of mine worked for internet companies a couple of years back. It was the big boom time in our area and anyone over the age of 14 could get a job with an ISP if he knew ANYTHING about the net.
I had a dial up connection at the time and had the same problems as you, always getting disconnected for “no reason”. Then I started hanging out with the guys at work and I got my asnwer…
They had some pretty shaky equipment back then ('97 I think, but I have a bad memory) and all the brandnew ISP companies were way too low on modem lines, this I knew 'cause sometimes I got a busy signal when I dialed up.
But my friends told me a few horror stories about low-budget ISPs. For instance, their connection always started to get lagged when all the lines were busy, so when they wanted to download something big (like gamez!) they would simply hit a button: “Disconnect random users” and entered a value between 1 and 9999! They even did this for fun sometimes, or to show off to people like me 
Of course they got lots of angry calls and made up answers as they went along; they ranged from trouble with the phone company to asking people to actually bring in their computers for repair!
It was handy for them at home too, when all the lines were taken they just called up work and had them throw a couple of dozen out, that way he was sure to be able to jump on to a modem line before the legitimate user could reconnect!
So, while I’m not saying this is always the case, I have some bad experiences with the official explanations ISPs give you about connection problems. I understand the equipment used is a lot different today and ISDN, Cable, DSL and T links are really the only way to go anyways.
Just don’t be surprised if you hear sniggering on the other end of the phone when you call up the greasy team of teens working in the service department 
— G. Raven
p.s. In case you were wondering, the companies all went belly-up since then, or merged into bigger blocks.