Lag on ATI Radeon HD 4850

[Vue]

Hi im having lagging issues with my 4850. When i first fire up the computer and go into a game right away, everything is fine for the first 10 mins or so. After that it starts lagging and i have choppy gameplay. For intstance in Warcraft III, I can play for about 5 mins then after that it stutters for about half a second then goes back to normal. This will continue and start getting worst as the game progresses. Eventually it will start rendering like it is a low end graphics card. This card should have no problems running this game on high settings and thats what i have it on. I also tried it on Starcraft II and at first it was fine but after about 2 mins the game becomes choppy. i know that this card can play Starcraft II no problem because when i first start the game everything is smooth. My computer specs are as follow:

Intel Pentium D 2 core cpu at 3.0 ghz
400 watt power supply that i installed and replaced (Had a BFG 550W but the fan stopped, problem still persisted with this PSU)
4 gigs of RAM

I had a Nvidia Geforce 9800 GT before this card and it ran really good. But the stock cooling system was not effecient and it overheated. I can still run it on the pc but when I try to play games, the computer freezes and goes to a blue screen with some text that disappears too fast to read. The 4850 ran smooth on another pc that I use to have. I would also like to add that this card gets pretty hot, about 60c idle and 70c to 80c at load depending on what game I play. I don't know if it is a heat issue or not. The drivers are up to date on this card. Please if anybody can solve my issue or have any suggestions, it would be greatly appriciated.

 

[Rockdpm]

the 4850 is quite aged by a few years..may be time for a upgrade

 

[Onus]

This sounds like an overheating problem; starts out ok, then begins to lag as it heats up. Make sure there isn't a buildup of dust in your CPU or GPU coolers.
Although your parts are very modest by today's standards, that would not explain older games like Warcraft III lagging the way you describe. Rule out thermal problems before allowing yourself to be convinced you need to upgrade (unless you have other reasons for doing so).