judging from your screen name if it is an 84, I don't know if it has the valve in the right exhaust manifold or not like the older ones. Sometimes that exhaust valve closes and stays closed and under load, you simply aren't going far without all kinds of weird stuff going on--too much back pressure.
Are you sure your timing is correct?
SgtDel
aka "Andy"
www.delbridge.net
1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4