Runs erratically If I have it in park with no foot on the gas pedal it will rev up and drop down to normal continuously. Driving down the highway it will lose power...almost like its coughing or can't get enough air?

I assume the car you are talking about is your 1968 Dodge Dart GTS with a 4-speed manual transmission. The engine has a special camshaft, Chrysler P/N 312044, which has a 60 degree overlap in the valve timing, since the engine is built to rev in excess of 6,000 RPM. The problem with your car is obviously a leaky accelerator pump. Why don't you get a car that (1) does not have to run on premium gas (because of its 10.5:1 compression ratio), and (2) uses fuel injectors instead of a wild 4 barrel carburetor made for 1/4 mile drag racing? I happen to know that that is the car you are referring to, because you did NOT say that it WASN'T!

I would suspect a clogged fuel filter. They normally go bad above 60k miles.

I would consider changing the vehicle speed sensor. When they go bad, they can really throw out a lot of bad data to the computer, resulting in unexpected behavior like you're experiencing. They're only around $20-$25. I hope this helps! :)

Shuts off, or stalls when letting off the gas pedal? WIll it run down the highway ok?

fuel vent canister especially if you OVERFILLED the tank

The most common causes of this are a engine vacuum leak and a sticking idle speed control air bypass valve.

sounds like you need to replace the temp sensor

cadilatic convertor

You need to check the cable the runs from the gas pedal to the engine, it might be rusty and need some drops of penetrating oil at the point it meets with the engine accelerator valve.

Try changing the map sensor it controls idle and it could affect your car like that...........