Not Running Right On Long Trips and/or lower elevations.

[bordin34]

So a couple weeks ago I drove my car down to Englishtown,NJ (50ft) from my home in Mahwah,NJ (350ft), about a 2 hour drive down the highway. When I went to race on the 1/4 mile at Englishtown, the car pulled normal in first then just died and sputtered in second. My 1/8 and 1/4 mile mph were the same. Also the car didn't run right for the rest of the night in any gear really. It ran as if somebody decided to play with the timing. On the way home, the further north I drove, higher elevation, the better the car ran. I wouldn't think that only a 300ft change in elevation could make the car run worse, but I was only running #65 jets in the primaries and #68 secondaries. The next day I changed the primaries to #69 and the car actually ran better and was more responsive. I thought I found part of my problem.
Fast forward to this past weekend, I drove from my home to Atlantic City, about 2.5hrs on the highway and again the more South I drove the worse the car ran, to the point that if I really tried to accelerate at all the car misfired and stuttered the whole time. It would however run right at a cruise rpm with little load. If I had to accelerate I would need to drop down a gear to avoid the stuttering. After the car show was over, about 5hrs, I restarted the car and it ran just as poorly. On my ride home it seemed to run better the more north I went, but it still was stuttering a bit when I got home. To top it off I only got about 8-9 highway mpg the whole time, I usually get 15 highway mpg.
Any ideas?
The motor is a 73 400, with a '70 383 4-bbl intake, 670cfm Holley Street Avenger, no vacuum advance, a mild cam, 727, and 2.45 rear gear.

[MSRacing89]

Check the obvious things like a plugged fuel filter or your timing.  Sounds electrical to me.  Maybe your timing is off or you coil is failing?  I just fixed my friends truck that was doing the same thing, his distributor came loose and retarded the timing so much it would not run out of 1st gear.