My computer's pretty old (AMD Turion) so it was a thought - especially when streaming videos. I was wondering if at higher res the computer just couldn't keep up.
Turion isn't that old. However, streaming video is almost always Flash, and Flash is very CPU heavy, however I don't think this is a PC hardware issue, if a hardware issue at all. I've got a much nicer machine (figuring your Turion rig vs my Core2 Duo rig (no offense or anything), and I get streaming video issues from time to time too. It happens, we live with it. If it's happening all the time, then something is wrong. I have cable and live in a decent-size apartment complex, so during peak hours (evening hours), my internet becomes very slow. With DSL, the speed is fair consistent (as far as I know, IE it isn't too affected by others on the same segment), but rather the distance between you and wherever you're getting the signal from is what's important.
Streaming video is all about bandwidth. You're almost guaranteed to be bottle-necked by bandwidth than your computer not having the power to process and display the video, since your machine isn't that old. Pentium 2 is old.
Yes - sometimes it just crawls to a clunk - hard to enter text in remote boxes on web pages, streaming video locks up and then catches up etc. Sometimes I'll see a notification box saying "your wi-fi's connected again" (paraphrasing).
The notification you're seeing about your WiFi being connected again is the key to your problem there. Somewhere along the line (and it's a rather short line too, just between your WiFi adapter and your router), your connection is becoming intermittent. I've had this problem whenever I'd had 802.11g networking going on. Just going up one floor can hose your connection signal. It's leaps and bounds above B, but it's still perfect for an extremely small, single story, 1 to 2 bedroom house/apartment. If you're trying to get signal through multiple walls, forget about it unless you've got some serious antenna action going on.
One thing I'm curious to know is if you've tried this on more than one computer. This is a laptop and others are using wireless. Are you having the same issues on the other laptops? The Wii? The iPod touches?
How could I prove this? (to my ISP)
You don't have to prove it. If they ask for proof, you tell them to fuck off and demand to speak to a manager. Tech support is just as much about being techincally inclined as it is about customer service. They might ask you for screenshots of an error message you're receiving, but that's not to prove there's a problem.
Calling your ISP when you're experiencing the problem is only so they can check if they see any issues from where ever the hell they are and your modem. They can't really be expected to support the use of every router, switch, hub, and adapter under the sun, so they may refer you to Geek Squad or someone if they believe the issue is on your side of the router.