Visiontek Radeon HD 4870 Crash

I'm having a serius problem with my card. When trying to play or run any 3d app with mid to high requierments, my PC restarts itself.
I read the GPU temp with GPU-Z, and nothing goes beyond 60°C (with fan manually set to 70% with CCC)when it restarts.
I also tested CPU with Core2MaxPerf and prime95 for a long while and it run ok. But when I try to play MassEffect, Crysis, Prototype, or benchmark with 3DMark, the problem appears.
I didn't happend while playing some old games like Half-Life.
Tried with video drivers from 8.9 to 9.9, and with every motherboard BIOS version in ASUS website (the problem appeard a few weeks ago and I have been running the system for about a year now, with some unnecesary BIOS updates in the middle...)
I'm not overclocking now, never did it with this video card.

My system is:
PSU: Powercooler PS-850HE 850 WATTS
Motherboard: Asus P5E Deluxe
CPU: Intel Core2Duo E8400
RAM: 2x1Gb OCZ Reaper 1066
GPU: Visiontek Radeon HD 4870 512Mb
OS: Windows XP SP3

Random reboots under heavy system load (GPU + CPU working hard) generally point to a Power Supply issue. Your PSU might be degraded, or faulty. The best first test would be to try another PSU.