Since the I've had the car ('98 Stratus, 180,000+ miles now) the
cruise control has intermittent failures. The ways it happens are:

1. After start driving, turn the CC on, the CC light comes on. Pressing
the set speed button makes the light come off. Can't be turned on again
unless I shut and restart the engine. Usually works fine after the restart.
2. Having driven a long time, with CC working fine, come into city
traffic, have to brake/accelerate. After passing through the city
traffic, want to resume cruise, pressing the resume button shuts the CC
off, as above. Again restarting engine makes it work fine.

These effects are more pronounced in warm weather; in the winter it
happens very rarely; in the summer - almost daily. Something else -
after each "major surgery" on the engine, the effect disappears
completely for 3-4 months. After changing the head gasket, the rear main
seal and the cat converter (each on a separate occasion) the CC worked
fine for the next 3-4 months. Each time I was hoping that it was fixed
permanently, but it came back.

Now the warm weather has returned, and it's happening again. I would
like to figure out why it's happening, if not fix it, before the car
eventually departs (it won't last forever, unfortunately). The several
attempts to diagnose it at dealerships have failed - it's working fine
when there, and the scanning tool shows nothing (admittedly that's what
they tell me - I have no idea what tool they use and which parameters
they read).

Can somebody (maxpower?) suggest a sure way to diagnose the reason? Do I
need to go to a garage right after it shuts off, with the engine left
running? (Usually that's impossible, because it happens on the road far
from known dealerships/garages and I can't drive the rest of the way w/o
CC). Do I have to tell them what specific scan tool to use or which
specific parameters to look at? Or maybe something else to look for?