You've got gasses going the wrong direction (getting to basics here). What that means is you've probably got a valve theat's open at the wrong time. Since, based on your rpm qualifier being correct, it seems to be "rpm based", it's either a timing or mechanical problem. If you had a "bad grind" I would think you'd have trouble across the entire range. Could be you have a bad or broken valve spring. Since it's coming through carb it would have to be intake. Could still be the dist. Based on the way you phrased the original post it sound like it did this before you rebuilt the distributor. Yes? Might also examine the timing chain/gears.