During the past few days my son's 78 bus has been running like
crap...misfiring, backfiring, etc. at mid to high revs...lack of power.

So I changed the fuel filter...it ran ok for a day or so, and then it
started doing it again. To day I broke down and bought a brand new fuel
pump...put it in...

It ran well for about 10 minutes...then it started the same old crap
again...it idles fine, but as soon as it gets within 750 rpm or so of
shifting to the next gear....it dies...I shift up to the next gear...it
catches and runs until its time to shift..then it dies, etc.

It seems like when there is a high demand for fuel is when it dies....it
also starts backfiring   just before it dies.

I think the problem is there is not enough fuel pressure to handle the
load...which leads me to believe the pressure relief valve is to blame.

Opinons? Any ideas? I drove it to the repair garage to night...barely made
it there..by the time I got there, the only time it would run was when it
was on idle...otherwise it would only run while starting and only for a few
seconds after that.

Any help, suggestions, etc. would be appreciated.

Thanks.

Ric