6.0 down on power....sometimes

[Levi Henson]

Just bought a 2003 GMC sierra 1500hd 300k on the body but paperwork provided from the owner says it had an engine swap with 140k on it. Sometimes pulling hills ill lose speed and have to floor it to kick it down to gain speed. engine has good compression and other times performs fine. MIL/CEL never comes on so I assume there isnt any codes to read. What can I check? the exhaust from the cat back is like swiss cheese which I will be replacing tomorrow but I dont think that would cause significant loss of power would it? Manifolds and stuff are fine no leaks. 

[sefiroxx]

ECU uses tps, rpm, speed and gear to choose timing of gear shifts. Use a scanner to check readings and work back from there.