Pc hangs for different reasons

Hi Everyone, thankyou for reading this. pc is athlon 1.4 mem 512 ddr, geforce 3, usb acatel modem win98. I’ve tested (swopped in and out) memory, video card and harddisc and psu. still the pc will hang while doing anything, even browsing, I am a technical support personage and hold my hands up to this one, I have run out of ideas, any help greatly appreciated, thankyou.