Rough idle, no codes and no warning lights

Hi there. This problem has plagued me and left two mechanics stumped.

A few weeks ago I was sitting at Sonic with the engine idling. It was about 20 degrees so I left the car on so I could use the heater. The RPMs started to dip, then go back up, then dip, then go back up. I backed out and started to drive slowly and it tried to die, but kept itself at 500 RPM then recovered. I parked and it continued the dipping and recovering, never going below 500 RPM. Drove it home shortly after and didn't have any trouble until two days later. It didn't store any codes and no warning lights came on.

Cut to two days later. I was waiting in the left turn lane and without warning it died. I tried to start it up right away but it couldn't turn over (the not starting part probably doesn't matter because the line was starting to move and I panicked and tried to restart it while it was still in D). I popped it into neutral and it started right away without any problems.

I took it to an independent Volvo mechanic. He replaced the flame trap, cleaned something on the invoice listed as "carb choke", and replaced the vacuum hose.

Got it back. Worked fine until I was sitting in my car before class and it started dipping and recovering again. Keep in mind that it's only died once at this point. It kept doing its thing for about 7 minutes and then it did something slightly different. It started shaking in a fashion that I'd call just a hair short of violently. I shut it off right away. When it was shaking the RPMs didn't fluctuate more than 50 RPM in either direction. Everything was shaking; wheel, dash, mirrors, etc.

Back into the shop it went. It finally died when the mechanic had it (it wouldn't do it the first time in the shop). He checked all the obvious things. Fuel pressure was perfect (even when it died), all the spark plugs are functioning perfectly (replaced about a year ago), no clogs in the fuel lines, no vacuum leak, checked the ignition system (didn't mention it so I assume there was not a problem). He ended up replacing the idle motor with a used one. Earlier today when I picked it up he said he let it idle for about 40 minutes and it didn't do anything bad.

Got about 8 miles before it started to act up again. Got home, parked it in the driveway, opened the hood and let it idle. Went inside for a minute, heard it die, came out and tried to restart it. It took 4 good tries to get it to restart. It would turn over, act like it was going to start and then die, but not abruptly. It kind of "floated" to a stop. Did that two more times and finally started on the fourth try. When it was restarted I revved it to about 4000, let off, and on its way down it popped a little. Did it a few more times and the same popping noise. It wasn't LOUD loud, but definitely noticeable. I don't think it was a true backfire (I've had it explained to me the difference between a backfire and something similar and much more frequent that people call a backfire. I've had real back fires before when the spark plug caps failed).

I took it over to my fiancee's house so her dad could look at it. He's been a Honda service director for over ten years so I figured he might be able to think of something the mechanic missed. Had the car idling for about ten minutes before it did the dip and recover thing again. He couldn't find anything obviously wrong with it. He mentioned a sensor might be failing intermittently or the fuel pump might be failing, though as I said before, the mechanic checked the fuel pump and said it was fine.

Remember that through all of this no warning or check engine lights came on at all.

I drove it to the store today and it stalled once (not in traffic thankfully).

This might not have any influence over the actual problem, but it seems that every time I pull into my driveway the problem gets worse. It may just be because I slow down to a crawl to navigate my garage, but today it was dipping and recovering. I turned it off to see if it would start again without the problem and this time it took 6 cranks to get it to work again.

Any would would be greatly appreciated.

Thanks,
-Mike H.