You probably don't want to hear anything like this from me, but I had a similar problem with my old '87 when I tried to pull a 95 extended cab truck with it. It couldn't hardly hold 65, it freaked out at signs of hills (going from palm beach to pensacola) and would lose speed pretty badly I was towing in 2nd gear almost as often as I was in 3rd. Any time I tried to accelerate, it would heat up extremely quickly. This was with a C6 and 4.10's so you'd think the truck would laugh at an 8k load. I thought I had a bad head gasket, because I knew I had two broken head bolts. Turned out that the whole engine needed rebuilt, because I could move the pistons back and forth inside the cylinders. Hopefully you don't have my luck when it comes to trucks, and it is just your IP and injectors.