My 850 GLT Wagon, 5 speed, conventionally aspirated, with about 135,000mi has developed a random bad habit. It sometimes stalls at idle, more often when you pull to a stop, seldom at speed since normally one puts the car in gear and engages the clutch. Occasionally, sitting at idle, the rpm becomes unstable, revs and drops below normal, very occasionally this 'hunting' will result in a stall. Once, only once, the car stalled at a light, was hard to restart, commenced to run very badly with no power and lots of backfiring and a bad exhaust smell; this time, the car would idle; when I checked under the hood the exhaust was cherry red and the surrounding insulation was smoking hot; three hours later, having had the car transported to the dealer, it ran perfectly (and for a month afterwards!). Once, recently, the car 'stuttered' under load; no repeat, no stall.

The car has been repeatedly checked at the Wynn dealership near Philly. It has never shown the 'check engine light', nor any codes. They cleaned the throttle body but no change was noted, either in the occasional rough idle or in the random stalls at a stop.

Any ideas?