is my trans taking a dump?

65 valiant slant six. 904 auto.
Engine is brand new and runs very smooth in park or neutral. Once put into gear it seems to jerk more than it should into gear. And car turns from smooth to pretty bumpy. Car will shake and feel a bit rough. Only during idle. When taking off it has a flat spot that I cannot seem to get rid of with any kind of adjustment I make. But after that small flat spot it drives how it should. And problems seem to only occur during idle in gear. Is this a trans thing or torque converter? Fluid level is good during hot neutral check.


Do not know history on trans. It is a bit dirty and seems to have some sweat like residue from atf.