Inspiron 1720 unexpected shutdown

My 1720 has experienced unplanned shutdowns since about the time the warranty expired.  I've searched the net for answers, tore it apart, cleaned everything and applied new thermal paste.  Checked for drivers, dumped unused files, ran Dell diagnostics, got a cooler, installed a fan control program and lots of hair pulling.  I recently installed Win 7 and it still shut down.  The CPU temp tracking never showed a too high core temp.  It seemed to shut down mostly during streaming videos or intense digital image processing.  I was about to give this clunker a pinada party send off when I tried one more Dell diagnostics that came up with nada.  I went thru the bios settings and decided to disable Speedstep.  Success!  Cooler pad now gone.  Fan speed back to normal.  I ran streaming Youtube music videos for several hours and then installed Quake III and ran the heck out of it.  No shutdowns in three days of hard running.

Can anyone explain Speedstep relationship to shutdown problem and has anyone else had shutdown issues that they fixed and how did you do it.

Thanks,

Jazzmaster