Hmmm I have a 79 TransAm with the 403 also and have not run into this problem, they are really good running engines. It runs fine otherwise?
I have had my timing off both ways but never had a problem like that, but chack it any way. My guess would be somthin wrong with the carb. Do you have the original Quadrapuke er Quadrajet carb on it
mine was absolute garbage, It didn't do what your describing but it made the car run so bad that I went and got an Edelbrock carb and adapter for $260 from napa and now the car feals like it's fuel injected.
Well don't know if this helps any but its all I've got for ya. let me know if ya figure it out.
Good luck