Oldsmobile/Buick Repair: 87 R10 with 305, vacuum leak, chevy pickup

Question
My Chevy pickup will run fine sometimes, however, more often then not the engine will start to run rough and stall. During these times the engine will have very little power. Usually when it stalls, it takes a bit of effort to get it started again. I've replaced the fuel filter and fuel pump. A mechanic had told me it may be the ECM causing this, any thoughts?
Thanks

Answer
87 is in the time when carburetors were used. Do you happen to have one?
If you have fuel injection, I would still check the pressure to make sure it is up to spec. Especially if you can have it connected when it dies.
A gas cap not letting air into the tank could starve the engine.
Another thing that would cause it to act like that is the egr valve closing on some carbon, which would be a large vacuum leak, and it would correct itself when it opens again and blows the carbon through.

Van