Car racing when started

This car ran perfectly until a month ago then started racing and stalling. I have replaced the IAC three times all AC Delco from Rock the TPS twice also AC Delco and the temperature sensor IAC some times gives erratic readings but no service engine light or code shows. I then put it in learning mode and worked for about 12 hours then right back to the same problem. Replaced the ECU twice first one was bad same results checked the fuel pressure was low changed the fuel regulator the fuel pump and the fuel filter. Now right on 40 PSI and holds it. Checked for vacuum leaks found none. Sent it to two shops no luck. I know I am missing something hoping someone has a clue. I am a certified Marine Mechanic but this has me lost.