Hoping for some advice before I sink any more money on trial and error R&R !!

My non turbo 94 850 wagon with 205K miles suddenly began to idle erraticly and die frequently several months ago. It already had a very bad PNP switch at the time which made it very hard to restart which it required frequently. I had noticed some occasional "stumbling" at highway speeds prior to this which I suspected might be related to the PNP switch problem. I replaced the plugs cap and rotor and cleaned the throttle body and flame trap - all which appeared to be in need of service and noticed a slight improvement, but certainly not a solution to the problem. I then tried relacing both the fuel pump relay and the fuel system relay again with no effect. The combination of the idle/dying problem and the PNP made it nearly impossible to drive the car so I took it to my local indy volvo specialist. The car was setting multiple error codes at that time (don't have the list handy any more but I remember that they included 511 and 512) and I suspected either the O2 sensor or MAF but I didn't want to start purchasing these expensive parts on a trial and error basis. The indy mechanic diagnosed the problem as the MAF sensor and replaced it, then also replaced the PNP switch and fixed a broken hood release cable. He said he drove the car around for a half a day and had no problems but I couldn't make it home without it dying at nearly every stop - but at least it was easier to get it restarted with the replaced PNP - although it is still tempermental and I have to "jiggle" the gear selector a little in P or N roughly half the time in order to get it to crank.

I took it back and the mechanic then diagnosed it as needing an O2 sensor as well which he also replaced without any improvement. He also said he had tried swapping the computers with another 94 850 wagon he had but that didn't improve it either. Having already cashed the check for $800 that I had paid when he returned the car to me "fixed" the first time he declared that he couldn't figure out what was wrong with it and could do no more. He said he had done every test he could - the only other posibility he could figure was an electrical short somehere but that only a Volvo dealer would have the necessary tools to test it. I got him to take it to the local dealer for this test and agreed to pay for no more than one hour of diagnostic time there. A week went by and he told me that the dealer had not been able to diagnose the problem and that he thought the engine was just worn out and wouldn't run correctly anymore. He told me the dealer had spent over ten hours trying to diagnose the problem! I got the car back and it still runs the same -

It will always start right up , I retrieved codes 223 and 511 from A2 yesterday and reset it. It seemed to run pretty good for about twenty minutes after being reset then the problems return. The idle surges between 400 and 900 until it dies. Giving it some gas in nuetral will usually speed the engine up but then if I try to hold it at around 2500 RPM it will suddenly stumble and the rpms will drop to about 500 for a second ot two then it will die. Pumping on the gas pedal will sometimes revive it before it dies. When driving it will barely idle or dies when I come to a stop. When I give it gas to accelerate, particularly from a stop, it often hesitates for several seconds - sometimes as long as ten seconds, acts like it's about to die, then suddenly "catches" and takes off like a rocket. Once it is going at highway speed it does pretty well running smoothly and powerfully although I feel like I have to drive it "heavy" - giving it just a little gas will tend to make it stumble while pushing hard on the pedal gets instant powerful acceleration. It does do better if I keep the gear selector in 3 instead of D and the pattern selector in sport rather than economy as it seems to hesitate if it is in the OD gear and I attempt to give it some gas - it bogs down as if the brakes had been applied for a moment then downshifts and takes off. It doesn't have this problem if I keep it in 3.

The car does have a leak in the gas tank when it is filled up I discovered a few nights ago - I don't know if that could contribute to the problem but I mention it only because it could cause a loss of pressure in the fuel tank - but I know the car ran fine for a while in the past when my wife left the fuel cap at the gas station until I retrieved it. I did recently flush the transmission and the fluid I removed was really bad - the transmission shifts noticebly smoother now but no effect on the dying/hesitating problem.

I am now thinking that the throttle position sensor or Idle air motor (which I have cleaned and does vibrate when the key is turned on without starting the engine) might be the culprit but am hesitant to keep buying more trial and error parts!

Any thoughts or suggestions appreciated!

Sorry for the length of the message!

Jim