1999 gmc suburban looses power above 1500 rpm when towing a medium load?

[Dale]

This happens after about 2 hrs of drive time. When it rests for 10 mins it's ok for another 45 mins. Couldn't it be a c.converter???

 

[Molson02536]

This happens after about 2 hrs of drive time. When it rests for 10 mins it's ok for another 45 mins. Couldn't it be a c.converter???

Hard to say without having the PCM scanned for any DTC codes related to the transmission. It could be poor line pressure rise due to leaking boost valve, clogged EPC filter screen, failing EPC solenoid, or worst case: leaky seals throughout.

Put a line pressure gauge (or visit a local trans shop) on the pressure test port and go for a drive to verify proper line pressure. Base pressure at idle in P,N,OD should be around 70psi and around 140psi in R. Max line pressure should be at least 190psi at WOT in 1st or 2nd, 225psi is optimum. If base pressure is low, trans or pump is worn out. If max pressure is low, check EPC solenoid. If reverse is slow to engage, and line pressure does not come up to at least 130psi in reverse, replace the boost valve with a Transgo 0.490" boost valve (easy fix in pan).

Good luck and keep us posted