The day before yesterday I drove my 92’ 940 turbo (137,000 miles ~220,000 km) about 2 miles (3 km) and stop for an hour or so. On my way back home the car started to act strange. It started fine and drove O.K. till the RPM reached 2,500 ~3,000. The engine lost power and the RPM dropped to 1,500 ~ 2,000. When I let the pedal, RPM increased and everything seemed to be fine. There was just a threshold I couldn’t pass to keep the car behaving normally.
{Back in Europe I had BMW 520 with carbureted engine with aftermarket propane installation. There was a tree position switch inside the cabin; first- the car was running on gasoline; second – cut off the gasoline, engine running waiting to burn out the gasoline, and then when the engine choke (no gasoline) the third position-propane. So my 940 was acting as if it was starving for the fuel waiting to “switch it to propane”.}
I couldn’t accelerate going uphill, just holding the throttle pedal pressed a little bit and not going over the threshold. The drivers behind me in line was thinking “must be a grandpa on a stroll”.
Came back home, checked the codes. 2-3-1, O.K., but I use to have it from time to time since I got the car (over three years now) without any symptoms. Well, there is whistling sound on acceleration but I couldn’t find air leak with “spray propane trick” so I left it like it is. Anyhow, I used my wife’s 740 turbo to swap parts, started with power stage. No improvement. The car runs, but this time dies on the street when I came to stop. The next day, yesterday I got ready to more swaps, but right before it I said: lets give it a try. Same story, but this time car idle at ~1,300-1,400 RPM and eventually dies. Pressing the brake pedal, shifting the gear lever or any attempt to move the car was speeding up the process of engine shut off. O.K. one more try (I did nothing, just turn the ignition key one more time). Alleluia!!! My brick is back! I run it around two blocks at my place-everything is perfect!!!

For how long? Something is breaking down here, what it could be?
Last summer: new timing belt, new tensioner, new front seals, new water pump, new fuel filter.
Almost two years: RPM sensor, flushed ATF with Mobil 1, original Volvo coolant, TB and IAC cleaned, new ignition cables (Bougiecord).
Almost three years: check valve at the pump, new distributor cup and rotor, new NGK spark plugs.
Over three years: new radiator (Volvo), upgrade to coolant level sensor
The car is running on Mobil 1 10W30 summer, 5W30 winter

So my cry for help is: does anybody knows what could it be? Did anybody experience anything like it? What is it? Could it be something in the gasoline? ECU is going south? Bad FPR? IAC? TB? TPS? Electric harness ? Bad ground?
Now the car is running, but what if it come back thousands miles (km) from home, and this time for good.
Thanks in advance.
--
'91 740 turbo SW, '92 940 turbo (PL)