constant freezing

Hi there
my system is as follows
Athlon XP 2800+ Thoroughbread Rev B
768mb Unmatched DDR 266 ram
Windows XP SP1 + All updates etc
Geforce 4 MX440 PCI graphics (crap, i know... but getting the job done till
i get my new graphics next week)
Jetway N2PAP 400 Ultra motherboard
all laterst nforce

my problem is that im getting intermittent freezes in games.
ill be merrilly playing along and usually within an hour of playing my
machine will freeze up... the screen will stop dead and nothing but pressing
reset will revive it...
occasionally it will blue screen or reboot... if it blue screens it ses that
windows detected a problem in my hardware and stopped to prevent damage to
my hardware. underneath it says irq not less or equal normally

this will also happen more reliably if i start stressing the cpu for more
then a minute or so...
i can make my machine repeatedly freeze in distributed.net cracking program
within 1 or 2 minutes of running it

recently i did have a powersupply burn out on me... it happened after i
pushed my graphics card to a much higher overclock... i dont think my
graphics card is overheating or anything tho because i have customized its
cooling and it runs stable in another machine... after the burnout i have
replaced the PSU temporarilly with another 300 watt but this is temporary
untill tuesday when i have a 550 watt ordered.

i have tried another graphics card in here (a freinds radeoon 9600 xt) and
my machine freezes with that card also but it seems to take longer oin his
card... i havnt tried stressing the cpu with his card in my machine tho.

i have however noticed since changing psus i get more freezes much
quicker... the new psu doesnt provide as much power in amps to the
different rails

i dont think my cpu is overheating because it is allways running at
50-59c... and never any hotter
never the less i have got a thermaltake volcano 12 heatsink and fan on the
way with a new psu on tuesday...

any ideas as to what is causing the freezing?
dr watson shows nothing after a freeze

alan