I have an intermitten problem with engine, sometimes have power and torque and sometimes don't. Wut could it B?

I've rebuilt the Throttle Body and cleaned real good,didn't help. I've replaced TPS, didn't help. I've replaced the distributor and it helped for about a week, but back to same ol problem. Its a 1991 Chevy Suburban 1/2 ton. Oh and before I forget, I did all the regular tune up before I started the other stuff mentioned earlier. So wut could it be? Can any one help me with this problem? I want to just reuild it but don't have the cash right now, in a recession you know...lol.