I inherited an '04 Seville. N.*. engine. Surging at idle. If I disconnect the IAC it runs almost right.Idles high but won't die. Put in new IAC and still surges. Will surge up to about 1500 then down very low. It gets lower and lower each time till it dies. It will start right back up.

If parked with nose downhill surge is worse. Park uphill and it idles fine. Put in new fuel filter and cleaned TB. Also has a new gasket between the TB and plenum.
Has new MAP sensor and MAF and TPS. Also checked the fuel pressure valve but not the pressure.

It drives fine until coming up to a red light or turning a corner, then it dies before I'm stopped.
Throwing no codes. About out of ideas.

Check fuel pressure with key on engine stopped. Come back here with numbers.

Check fuel pressure with key on engine idling. Come back here with numbers.

Check vacuum while this is happening.

Is the check engine light on?

Are any codes stored?