Loss of power under a load

I have a 1999 GMC Jimmy, Going down the road it does fine. But, when it has to go up a slight hill or a big hill, It starts to trip all over itself. I pulled the fuel filter, good flow and clean. Cleaned the MAS still nothing. Replaced the Cat. Converter, still nothing. Replaced the MAS and still nothing. Can you tell me What else I can check or fix to get rid of this problem?

Basics, how much fuel pressure?