I have a 2004 fx4 scab. For about 2 months now. When the rpm's reach 1800 rpm, the truck bogs down and has no power. When the rpm is back up to 1900 and above. Things are fine. Same when the rpm's are below 1800.
When this happens, the vibration is very loud. I have to either accelerate or drop the rpm's. Any ideas? The truck goes in the shop on june 23/08.
Thanx guys.

 

Hmmm...If it were a diesel, I'd check the speed sensor on the xmsn for proper adjustment. If it were a gasser, and had points, I'd check the distributor timing and point set, or look for vacuum leaks.

Check your COP's....