Got an odd issue in my 1977 Corvette with now only 43,984 miles (no modifications done to motor except headers). I have a pretty steep driveway and every time I drive up it in first gear (at low speed, not driving it hard), I get a significant amount of blow by coming from exhaust. if I go back out later, no smoke/blow by whatsoever upon startup. 

It will also occur very randomly at a stoplight after I have been driving it for about 30 minutes. The most consistent time it occurs is going up the steep driveway.  Any ideas of what this could be and why it is so random?  The intake gasket was replaced by previous owner a couple of years ago--maybe its not sealed all the way at some point? 

Worn valve stem seal on one of the back cylinders maybe? Oil flows back there on a steep incline?

Thank you--couple of others have said that as well from a recent car show, so good to hear confirmation. That's the first place I will look then. Thanks again/will report back once I get to it. 

I agree with the diagnosis of valve stem seals. In order to do it on the vehicle you will need an air compressor and adapter to fit in the spark plug hole to keep the valve from dropping into the cylinder. You'll also need a valve spring compressor to compress the valve spring in order to remove the keepers.

You can also use a piece of rope.  Shove it in the spark plug hole, rotate the engine to jam it up against the valve, change out the seal, back up the engine slightly and pull out the rope.  Works great if you don't have the tools for the air compressor procedure.

P.S. take alook at the p.c.v. valve and the p c v hose . Bob.