low speed stall and also speedometer bouncing 10-15 when over 40 - SOLVED

97 Buick Riviera, non supercharged, 200,205 miles.

Hi guys,
I have owned this riv since June 2014 and 186k miles. Except for requiring a hub and suspension work when purchased, it has always run great. But since hitting 200k on Monday, I have had random low speed stalls once or twice a day. Since the first stall, I also have a problem with the speedo. Once i am driving over 35 or 40, the speedometer sometimes bounces in a 10-15 mph range, mostly only after taking foot off accelerator. But once while accelerating to 70 on the freeway, the speedometer was bouncing between 75 and 90.

Conditions on day of first stall were: full tank of gas, warm engine, 12F outside, 12" of fresh snow on the ground and very snowy roads.
1st day stall 1: Turned out of store and accelerated to 20 on snowy road, let up on accelerator to change lanes through a snow pile, stalled.
2nd day stall 1: Cold engine, drove two blocks, let up on accelerator for stop sign ahead, stall.
2nd day stall 2: 3/4 mile later, stalled while at red light.
dry gas/fuel injector cleaner.
2nd day stall 3: stalled that afternoon on way home from work.
3rd day: drove with 2 feet at stops and di not stall
4th day: 1 stall in morning. put in 12 gallons of gas and another dry gas/fuel injector cleaner.
5th day (friday): stalled while turning into driveway.

After every stall but one, the car started right up. On the cold engine stop sign stall, it cranked for about 7 seconds before starting.

Any suggestions? Other threads here seem to point to:
Crankshaft position sensor, but my stalls are only at low speed and with warm or cold engine.
Ignition switch, but my stalls are only at low speed and with warm or cold engine.
Frayed random wires.
Fuel system.
Mass airflow sensor?

One more thing... The staling and speedo problems started at the same time. There is a third issue I noticed at the same time, but I can't say for sure that it started at the same time.
If I have the heat on, the car doesn't get up to operating temperature. Even on 20+ mile drives, it only gets to 125F. Outside temps have been between 7F and 25F.

Add Idle Air Control motor to your list of things to check. Otherwise, you already seem to have a good list of things to check out. I don't think CPS needs to be on your list, but wiring harness is a good possibility. Grounds are important here.

Getting up to temp would only be related to your thermostat and/or low coolant. Where are you getting the reading 125F? The gauge on the dash?

x2 on the IAC

Thanks, I will update this weekend.

I replaced the fuel filter on the day of my first post (Sat Feb 07). This seemed to fix the stalling problem, but not the speedometer issue. But 90 miles later the stalling returned and became more frequent.

Stalling, fixed.
Last Saturday I removed and cleaned the throttle body and honeycomb filter (almost completely clogged). I also replaced the idle air control valve. No stalling since.

Speedometer issue, fixed.
The speedo would act up on about half of my trips. I used the Torque app all last week, so that I could watch the sensor outputs when the speedo would act up. If gps speed was 65, speedo might bounce between 70 - 85, and the app would show speed output from the car was also bouncing, but a much lower speed. Like 35-45.
Replacing the vehicle speed sensor that is outside the trans has fixed this.

Thanks!

Fuel pump? My riv was acting crazy when the pump died on me.