Check with Nero Info tool, please.
The cables haven't been touched. As for the hardrive that hasn't been defragged sicnce I bought this PC which was years ago. Would that cause the fluctuating only when writing image files?
Jepp. This might be one reason.
I can burn data discs at full speed with no problems but when I burn an image file at above 2.4X the buffer always fluctuates at some point during the burn.
Then there are really some issues with your system. Your drive by itself is capable to burn the media at full speed.
Does the fluctuating casue any negative effects on game disc and is it possible to burn these discs at a lower speed?
I don't know anything about game dics, but you really should tidy up your system instead of burning at lower speeds, that aren't supported. Maybe there is some copy protection mechanism affecting your system performance. You should check that, too.
PCs pecs - P4 2.0 GH, 256 ram, 80 gig H-drive.
You also should get additional 256 MB or even 512 MB RAM. That also would increase overall system performance. Running XP using only 256 MB RAM is at the lower edge.