Occasionally, my 3.0 feels and sounds like it is giving up around 3,500 - 4,000 RPM. Usually it pulls strong up to 4,200 or so. Strong being a relative term of course.

But sometimes the motor will start to fall flat on it's face once around 3,500. Engine starts to make what can best be described as a rasping sound, hollow almost... nothing grinding or mechanical.

Any guesses? Engine does have to be warm, but the occurrence is somewhat random and not very frequent.

 

I'd start with a fuel filter and go from there. could be alot of things. start with the simple most likely first. Also when it does this stop and look at the cat. see if it's red or the pipe in front of it red. Could be the start of your cat getting plugged.
Just my 2c
Good luck

 

Another simple thing I've found to effect power in that range with mine is sparkplugs. Pull one and see if the electrode is rounded off at the edges, and check the gap if you want to reinstall it. Either one of those factors will cause inefficient spark, and some loss of power.

 

weasey's cams worked for my similar issue

 

Have you replaced the alternator lately? I heard something similar in mine at around 3500. I assumed it was the cheap alternator pulley.