ok the best way to get to the bottom of this
download and install this
RivaTuner v2.24 download from Guru3D.com
during install you will be asked if you want to install some riva tuner add ons , click YES , these are the signed drivers for vista
run it , under the "main" tab goto the FIRST customise tab (its like a little triangle) click it and you will get a popout list , click the last one it looks like a magnifiying glass
yuo will probably get a popup asking you if yuo want it to automatically install any plugins that will apply to you , click YES
a graph will pop up
at the bottom right click SETUP
under DATA SOURCES put a check (tick left hand side) next to...
core clock\rop domain
core clock\shader domain
memory clock
core temperature
fan duty cycle
sensor supply voltage
uncheck anything else
hit apply then OK
at the bottom of the graph there is a red dot (bottom left) you want to click this to enable background monitoring
you now have on screen everything you need to know about your gpu , now the fun bit
right clcik each graph one by one and clcik SETUP at the bottom there is an on screen display section
put a check in show ....... in on screen display
when you get to the last one before you exit in the on screen display section check the RUN SERVER option
click OK
run the game , you'll find all this info core clock speed , memory speed core temp etc magically appear in the top left of your gaming screen
you can then see EXACTLY what your gpu is doing , how hot its getting , if the voltage is dropping too low if its underclocking , what speed your gpu fan is doing etc
you can troubleshoot your issue from there
yuo can also set the size colour and position of the onscreen display , but im bu88ered if im going into all that now