Heat in the engine. My car will slow down a lot as the engine temp goes up. It runs good at 150, but slows down as it gets to 180, and then slows down a lot above that. If you made a pass and went right back and made another one without cooling off any then that might be the problem.

I wanted to say Merry Christmas to everyone who has helped me - but l've been sick with a head cold and haven't been going online. Thanks again for all the help.

Sorry to hear you have been under the weather. Hope you get to feeling better soon.

Just an update - after getting over a head cold, a few weeks later I came down with Type A flu, but I'm much better now and will be getting back to work on the Chevelle.
Still haven't made up my mind as to why the car slowed down. I know engine heat could do it, but the car was still slower the next day, when it had had plenty of time to cool.
I believe the old, used, engine may have just been going away.
Don't know if I'll get to the track to see what it does early this year, after making sure the rear brakes are backed off, or if I will just tear the engine down.

After looking at the engine, and finding a little bit of rust on the inside of one of the valve covers (there might have been more if the inside of the valve covers hadn't been chromed as well as they are, I don't know) - I've decided the intake manifold was leaking, and was probably getting worse. No water/anti-freeze showing in the engine oil, or the lifer valley. With this being just an old used engine, I have decided to just remove it, and rebuild it or replace it with another engine I get rebuilt.
Wish me luck - and better time slips in the future.

engine issues...
 

Couldn't you use a cooling system leak testing pump...the kind that goes in place of the radiator cap?? As for the motor...compression and/or leak down test might give you some helpful info.

Good luck...wilma

Thanks WILMASBOYL78 - one friend had mentioned the radiator cap leak tester, and I thought about it. But this engine is far from being a really good hp maker. So I've decided to come up with a fresh engine.
This one is a used standard bore 2-bolt main block with some used standard bore L-78 pistons from a different engine in it. That is not the best condition for making power.

Sounds like you got a back yard mutt build. Who knows what heads and cam too.
I agree, build an engine with modern technology and know what you have. The dome on those L78 pistons are antiques in the modern engine world. Most builders want flat or very little dome to achieve a zero deck height. Hydraulic, roller cams can perform better than the old school stuff too. 600 hp on pump gas is not uncommon.