Our 99 ml320 w/66,000 miles has started acting badly. My wife reported that a couple of times the engine speed was fluctuating and she couldn't get it to go more than 10 mph. When she got home the check engine light was on.

After letting it sit a little while, the light was on, but it ran fine. We
did take it to the dealer to have to code checked and they said there was fault code on a one of the cylinders. They swapped wires side to side just to see, and all was fine. They suggested that at the next service we get the plugs replaced ($385) because they should be replaced after 5 yrs or 100,000
miles.

Same thing happened a week ago. Wife reported it running bad and turned around and came home. It was fine by the time I got home.

Now today, we're backing out of the driveway and the low oil light came on. I turned it off, checked the oil and the level seemed fine. So we continued on our way to white trash world, and after about 10 miles and only 1/2 mile from WMart it started acting bad (finally for once while I was driving). The rpms would just go down like to was going to stall, then they'd surge a bit
and then down. We were able to get to WM.

I bought a quart of mobil1. When we started it to leave, it immediately stalled. Start/stalled several times, with the low oil light on. So, I go to put the oil in (just in case), and there was some tan goop all over the inside of the oil filler cap. I put the oil in, it started and did the rpm dance a few times and stalled. Started/stalled over and over. It finally started smoothly, and then we were able to get home with no problems.

About an hour later we get in it to go out again and it seems fine except the damn oil light is on again. The dipstick doesn't show low, unless I'm reading it wrong or maybe leaving it in too long. So we back out onto our road, put it in drive and it stalls. Same thing over and over. After 10 min I finally get it back in the driveway.

Any ideas what might be wrong? Sounds kind of (but not exactly) like some of the crank sensor or airflow sensor problems other have had.

I may have to get it towed to the dealer because I can't be sure it's not going to quit on me on a bridge I have to cross to get to the dealer.

I don't necessarily trust the dealer; i.e., I'm afraid that with so many miles on it they could say it's some item/reason that's not covered under the extended warranty and I'd have to foot a huge bill.