Overheating 390.

[Teslainnovation]

I have got a '69 Monterey (sorry, not a cobra) with an overheating 390, and lately I have had it. Sometimes seems like it has a lot power, other times it seems like my old man's compact Saturn! This seems to happen completely at random. On top of that, I now have an overheating problem. It seemed fine when my brother stopped driving it in 1999, but when I started driving about 1 year ago, it seemed like everything started to break. Last fall it began overheating, so I replaced the radiator and thermostat, and the problem was gone. Last spring, the troubles began again. I then replaced the thermostat again, along with the water pump for good measure. That seemed to work once again. About a month later, it started again. By this point I had no idea what was going on... Then I realized that my radiator cap was probably worn out, and... only 7lbs! I still don't know who put this in, and how my brother drove it without problems. I replaced it with a 13lb cap, and again, the problems seemed to go away, untill today. I got about 25 miles away from home, right in busy downtown Chicago traffic, when the lousy thing started to conk out. Yep, it overheated again. I have no idea what the problem could be. The heater core is also new, along with the upper and lower radiator hose. Does anybody have any ideas?
I figure it's time for a rebuild, maybe next year since I am not taking it down to college. Hah. Could I possibly stroke this 390 over with a 428 crankshaft, and some 4.23 " 427 pistons? I really don't know much in this area, so any info would be aprreciated!

Thanks,
Steve K.