My 1991 240 SE has about 175k.

When driving, the car exhibits an intermittent "hesitation" in acceleration, where it seems there is no power when pushing the gas pedal. This is intermittent, and on some short trips it does not occur at all. The feeling when it does happen is like the car starts to slow down, so I feel a slight jolt forward for the split second when there is no power in the pedal. I should mention that this hesitation will occur the same amount warm or cold, at all range of speeds, and on a full or near empty gas tank.

This problem is accompanied by an occasinal rough/high idle in P, and sometimes when stopped at a light in D, I notice a rough idle, and on some occasions the car will stop running while I'm stopped at a light.

I have investigated this problem a bit, though I am largely ignorant when it comes to diagnosing car problems, I hope I can learn a thing or two by looking into it myself. Upon discussing it with a good Volvo mechanic and having him hook it up to his computer, from what he said it seemed like it might be the air mass control. That's a 200-plus dollar part from IPD and I cannot return it to them if I buy and install and the air mass turns out not to be the problem. It was also suggested to me that I check my fuel pre-pump to see if that's gone - and that if I pulled my pre-pump fuse with the car running I'd hear some sounds indicating it's not working, but I tried it and did not hear anything (maybe I'm not doing that bit right though). I've also been told it could be the fuel pump relay, or the constant idle speed motor, but I'm not sure how to check these things.

I thought I might have some luck if I posted this little problem here, and if anyone can help a guy out, I'd appreciate it. I find that searching for info. on how to fix my problem on this site is a little hard to do, but if this problem is very similar to other posts, I firstly apologize and then ask if someone could direct me to the appropriate post or what search terms to use.
Thanks a lot!