CPU FAN FAILURE

I've experimented processor low speed (about 500MHz of 1.86GHz max on full performance mode) with apparently no crashes (now I understand some Blue screen days ago...) so looking for software problem or virus afecting this, some days after that, searching with NO results, I decide to run DELL DIAGNOSTIC bootable cd.

FAN TEST

Error : 3700:011B

expected high speed 4500, detected fan speed 2015. FAIL

I'm ordering cpu fan replacement.

IS THIS THE ONLY POSSIBLE RAZON OF CPU LOW PERFORMANCE? SYSTEM WOULD UNDERCLOCK PROCESSOR DUE WITH HIGH TEMPS TO SAVE HW?

WHY THE LOW PERFORMANCE STARTS SOMETIMES ONLY, WHY THE SYMPTOMS OCCUR EVEN BUT NOT EVERYTIMES AT COLD START?

It's oK to replace fan or better is save the money and look for a new laptop?

thanks in advance!

newros