Hi Porkface,

Today while parking the engine stalled, but it started right up.
Stalling at lights happened before about when it decided not to start.
I checked OBD codes and this time A6 2-1-4 RPM signal missing sporadically.
Another clue.

On a Hunch, I put a snap ferrite on the CPS harness and slid it down to the bell housing where the CPS shield is ungrounded. The ferrite is a choke to high frequency Trash/noise (EMI) from the ignition and alternator.
There's a ground wire to the bell housing but the CPS ground (shield)is not connected to the bell housing, which seems strange to me.
Gotta check that bell housing ground connection the next time the car is on a lift.

Volvo and Indy techs swapped out new OEM CPS several times to address the quitting without solving the long term issue. It's hard to fix something
that looks like it's OK.

Long time pro mechanics avoid after market CPS, maybe because after market CPS
use cheap un-plated connectors and cheap shielding. Quality of shielding in
coaxial cable varies widely and the good stuff, 98% copper braid, is pricey.
Couple that with weak filter capacitors (from aging) in the ECU and maybe that's what's going on. The RPM counter circuit may be upset with noise.
Why it works and suddenly quits in several month intervals is a mystery.
Resetting the ECU to fix is a big clue that the ECU is part of the problem
along with possible flaky inputs.

Best regards, Bill