[IXXBC]

Could my hot idle compensator be messed (do these even have those?)

The engine was freshly rebuilt, could my IAC still be messy?

I would like to know. :P

[DROPTOP STANG]

might even be something as small as timing... check that first

[TOM RENZO]

Check for vacuum leakk and do an minimum idle setting on your TB and TPS. Also make shure the iac is closing and in proper working order . Hot idle compensator. They went out years ago. They used to be popular in carb setups years ago. EFI systems dont use them. The ECM controlls all idle and mixture functions on modern EFI engines.