Frustrating problem - possibly vapor lock?

[grancuda]

I have had a frustrating problem for some time with my '72 I6 3on the colum, 3.73:1 rear gear. It is stock set-up with 89k on it. Here is how the prolem started out, about 6 months ago any time I go up a long incline over 65 mph the truck would slightly shudder, not really slow down any and you could accelerate but just you could feel something going on. This slowly progressed to about mounth ago the truck wouldn't go over 55mph and if you exceded that it would lunge really bad and shake the truck violently but on any semi flat/ barely incline it could go 70 before it did this. So a month ago when it got really bad like this I stopped off at Napa in the truck and picked up a carb rebuild kit,plugs, wires, condensor, points, cap, rotor, and some sea foam. I put the sea foam in at napa and started home, got about half way home and decided to try and accelerate up a big incline to see if the sea foam was helping any and it just took off like it was new so I figured the carb was plugged up so I put all the parts on and rebilt the carb. It did great for a month, 2,500 miles , no sign of anything then tonight it started it again but way worse, I couldn't exceed 55mph in the flat sections and 45 on I slightest of hills. I though maybe it isn't getting enough fuel but I can run it up to 45mph in 2nd, maintain that speed in 2nd for miles and it doesn't stall out at all. I can nail it in 1st, running it hard all the way up to redline then shift to second and run it up to 45 then go to third and it accelerates up to 55-58mph and starts cutting out / surging violently. I think since you can rev it freely/redline it in the lower gears it is getting plenty of fuel. What do I check next?