Originally posted by PurpleMystique:
..but why would it be exactly at 2500 and 4000?



Probably not always exactly at those RPMs but I would check the air filter and clean the MAF sensor.

http://www.contour.org/mods/mods.php?s=howto&displayid=36



"Always do the cheap and easy ones first." 1996 V6 ATX 96K miles