G5 PowerMac Intermittent cycling thru high to low fan speed

Lately, the fans on my summer 2005 Dual 2 G5 will rev up, run high for a few seconds and then slow back down. This happens for apparently no reason — even when Photoshopping or Video work, the fans didn't do this that I can recall. Have done several searches, and can't find any one fix that seems to apply. My 'puter's running Leopard 10.5.8, 4G RAM, ATI Radeon 800XT video card, 2 monitors,4 externall Hard drives, 2 on e-SATA, 2 FW.

Have done everything but reinstall (not sure that would help anyway), even started up from the HD for running Techtool, the fans rev up and down, tho not quite so frequently, and looks like when processor speed increases during the different tool functions.

Thanks for any help