truck feels like it is slipping at certain rpms
Hello everyone i am new to forums and online help. My issue is the following: my truck seems to slip after certain rpms, the funny part is that this happens every time i get near the 3000 mile mark on my oil changes. So when i change the oil this problem goes away completely and doesn't start coming back until about 2500 miles on the new oil. I have changed the rear main seal on the motor, have changed the clutch many times, and slave bearing. I have changed the transmission fluid many times and have had it checked by a transmission specialist, they said the transmission is not the problem. I have also tried different oil grades, from the standard 5w-20 up, 10w-30, 10w-40, 20w-50! The only thing i notice is that the thicker the oil, the longer it takes to start the slipping and jerking feeling. Also, the longer i go the less mph i am allowed to reach before it starts jerking again.
Truck info: 2000 3.0 flex fuel, manual transmission XLT with four doors. Rebuilt motor with 100k and 250k on the chasis (she is definitely a work horse).
Could this be a fuel delivery problem? or a camshaft sensor problem? also, nothing shows up on my check engine light.