1990 740 with B230F/Regina engine. Car seems sluggish, still stumbles a little when the throttle is suddenly opened alot.

I have cleaned throttle body and idle air valve. I have adjusted switch on throttle body. New plugs and plug wires. Cleaned flame trap. No OBD codes (except missing cold start valve, disabled per TSB a long time ago.) Constant Idle Speed system is regulating at 750 rpm normally now.

What is the next most likely culprit? How can I tell if the fuel pressure regulator is working properly without a fuel pressure guage (no place to connect one)? I read that pulling off the vacuum line should increase the idle. Makes no noticeable difference. Car drives same with this vacuum line pulled and plugged as it does with it connected as far as I can tell. When I reconnect the vacuum line to the fuel pressure regulator I get a momentary drop in idle speed and CIS then corrects for it. What happens when vacuum line is disconnected/reconnected on a properly running car? What difference is made in the way the car drives with it disconnected if the regulator is working?

Help, local shop is mainly good at miss-diagnosis and parts swapping. Dealer charges $$$$, takes two weeks to get a service appointment, and is not responsive to DIY'ers.