I'm running a '78 with a straight six and an auto TH350 transmission. She was my daily driver for three years until one thing after another started to go wrong. The past year I have been pulling my hair out trying to figure this out. I have a weak first gear it seems. She has trouble taking off but once I get to second gear, we are smooth sailing. Its a nightmare at red lights or hills. She would die while idling and at a light, she is trying to lurch forward and is very shakey. I took her to a local shop and they replaced some vacuum hoses and turned up the carburetor. She didn't die anymore while idling but the take off issue was still present, so is the lurching forward. I just got my '78 back from having the transmission rebuilt and the same issue is there! I have replaced the fuel pump, tuned it up, a brand new carburetor. If its not the transmission, what else could it be?

 

Sounds like it's time to take it back to the trans shop and get some warranty work done.