Ruscco,
I see some of the posts are talking about rust. I had the same symptoms as you are stating and after checking for exhaust gasses, changing a bad rad cap it got better but still iffy. Use vinegar treatment and now have a happy flatty.
Vinegar thread here:
https://fordbarn.com/forum/showthread.php?t=71022
Maybe worth a look at. Best of luck to you.
Fourdy