ATI Radeon HD 4650 underclocks itself and crashes

Question

My video card is underclocks its GPU, at idle it's 165 mhz, when recording screen it overclocks to 600 mhz (max 600 mhz). But when I play fullscreen games, sometimes screen flickers because GPU clock speed drops sometimes to 300 or 165 mhz. How to fix this?

Maybe my computer is too weak? My specs:

Hewlett-Packard HP d530 SFF motherboard
CPU Properties  
CPU Type - Intel Pentium 4, 2800 MHz (21 x 133)
CPU Alias - Northwood
CPU Stepping - D1
Package Type - 478 Pin uPGA
Instruction Set - x86, MMX, SSE, SSE2
Original Clock - 2800 MHz
Min / Max CPU Multiplier - 21x / 21x
Engineering Sample - No
L1 Trace Cache - 12K Instructions
L1 Data Cache - 8 KB
L2 Cache - 512 KB  (On-Die, ECC, ATC, Full-Speed)

Answer 1

It's possible that it is overheating. Check the CPU core and memory temperatures. The 4000 series were notorious for overheating.

I have two 4870s and I have six fans blowing at them to keep them to overclock.

Answer 2

The GPU should lower its clock only when it's underused. Did you try a benchmark or another game (more graphic-intensive)? Also, you can try to disable the power saving: How to disable ATi Powerplay

A workaround might be to limit the fps (if the game allows it).