I have a Volvo 1994 940 Wagon with approximately 180k that I've driven for the past five years. Over the past few months there have been various issues cropping up: wound up having the transmission rebuilt two months ago, which wasn't adjusted properly the first two times getting it back from the shop; after the 3rd the shifting issues were resolved; also my AC compressor is going bad and making a gradually-worsening whine when turned on, which also causes the vehicle to occasionally struggle to accelerate up from a stop.

Over the past few weeks, this problem deteriorated to the point where the vehicle frequently takes 5-10 seconds to rev up when moving from a stop. Idle seems relatively fine at about 1k, but about half the time it will refuse to punch through past 2k without hesitating and struggling. Gradually easing down on the peddle will sometimes avoid it, but as it's gotten worse it sometimes refuses even to do that. Turning the AC compressor on will almost guarantee the problem, but it still occurs frequently even with the compressor off.

If I shift into park or neutral and rev the engine, there's zero hesitation at all. Driving home today on the interstate in stop-and-go traffic, the vehicle performed mediocre for 30 minutes, then suddenly refused to accelerate at all past 2k RPMs / 15 MPH, even with the pedal held to the floor. Vehicle felt like it was bucking, and there was an occasional loud thumping noise from the engine compartment (engine light came on). I was forced to get off of the interstate and limp home on backroads, but shifting the stick into 1st or 2nd would seem to "break past" the barrier and allow it to finally make it past 2k RPM. Something very similar occurred two weeks ago, and we replaced the air mass sensor (Regina system). Things seemed slightly better up until today. My mechanic's only suggestion has been replacing the fuel pump, (and I have a replacement pump, but wasn't sure how to pull apart the fuel tank pump assembly) but I'm not convinced the pump is the issue, since it seems to be running fine at higher RPMs.

I'm not experienced in vehicle repair or disassembly, but if this is something that can be solved with simple tools and time I feel like I might have a chance...otherwise I'm not really sure how to go about diagnosing and resolving the problem. Any suggestions appreciated!