1998 BONNEVILLE SSEi, perplexing problems!

I have a '98 bonneville SSEi which has been giving me and my mechanic some grey hairs over the past few months.

Intermittently I've been having these problems over the past few (winter) months:

1) Start car with remote starter, runs fine for a couple minutes, then will begin to idle erratically. Revving, slowing, pumping out huge amounts of exhaust - some of which is is kind of brownish. If I turn the car off, it won't start and keep running unless I keep feeding it fuel. I need to keep one foot on the throttle and one on the brake to keep it idling. Check Engine light comes on, and it throws codes of P0101, which stated "Mass Airflow Sensor out of range"

2) Driving along, car's running fine, 15 minutes into my commute across town. Check engine light comes on - which happens often enough that it's not alarming anymore. I slow to make a turn and the car stalls (which is fun when the power steering goes out.). Same code (P0101). The check engine light does not typically coincide with any discernable problems like stalling. When it stalls, however, I have a devil of a time getting it (and keeping it) started. Once again I need to do the two-footed thing to keep it running while I put it in gear and throttle up.

3) Car starts fine, seems to run fine, but won't change gears when it should. It normally shifts between 2500 and 3300 rps (first to second). When it's "being ornery" (my mechanic's term) it will wait unti I hit 4000 or 4500 rpms before shifting from first to second, then will shift quickly into third and normally thereafter. It's fine between the higher gears, but if I stop and it drops to first, it repeats the process.
The Check Engine light came on again and it repeated the same code, but also did one (P0112?) "Throttle Position Sensor".

4) Fuel economy has dropped. Initially (Sept/Oct) I was getting around 23 mgp with premium unleaded, and that was with 70/30 city driving. After it started acting up, mileage dropped to around 16 - 19 mpg range, and hasn't improved substantially. I don't think I've been able to go through a whole tank without it acting up in one way or another, so I don't know how much the fuel economy is directly related to the other problems.

Each of these things (1 - 3) is very intermittent. Maybe one day out of 2 weeks of daily driving. It seems worse or more frequent when the weather is cold. The O2 sensor has been replaced, as has the Mass Airflow Sensor. The mechanic checked for air leaks and found nothing. He's stumped. The car had new plugs and wires installed when I got it (Sept. last year). It has 90,500 miles on it now. I love the car, but I'm tired of worrying about what's wrong.

This MUST be happening with someone else. Any clues? I'm just a li'l ol' blonde with not much mechanical knowledge, so please speak in terms I'll be able to comprehend, okay? I'm hoping to glean some info to help my mechanic (who is excellent, by the way) get to the root of the problem and fix it once and for all.

Thanks!!

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My thought is that it has to be something simple. Most likely the ECM computer is going out, battery/charging system faulty, or loose ground somewhere causing the computer to read uncorrectly.