Hello, I have a 1994 GMC K2500 with a 5.7L engine. It started out not being able to go up hills very fast. I could be going 60mph at the start of the hill and would be down to 40 by the time I got to the top. I thought I had a problem with my ECM and started checking things out. I found that the EGR valve solenoid was shorted and maybe that took out the ECM. I replaced the solenoid and replaced the ECM. The problem is still there. I have since replaced the fuel filter, EGR valve, Fuel pressure regulator, Fuel injectors, distributor cap and rotor and spark plugs. I am not receiving any fault codes. My problem is still there. Any help would be appreciated.

Thanks!

Sounds like your cat may be partialy clogged, not only do they fail over time but a poorly running engine will cause them to fail quicker.
A good muffler shop should be able to test it for you, If you do find that it is the cat going bad do yourself a favor and install a "high flow" unit, I had a "catco" unit installed but there are several others available, you'll notice the difference in the pedal and the pump.
One thing to watch for though is your o2 sensor, mine is located just before the cat and weather from the welding process for the newly installed cat or the clogged overheating cat itself my sensor failed within 50 miles of the cat replacment.
Hope this helps.

I also think the cat may be the problem, you also may want to check your timing. How may miles, could be getting some slop in the chain.