>This is a tough one to debug because the problem never occurs during idle 
>or engine revving and only randomly on the road...

Oh, I'm an expert at this now :-) You may want to wiggle all those wires
in the back while the engine is at idle. When it stalls, you'll know
you're on to something.

Could also be the connection to the cylinder head temp. sensor, which may
seem positive even when it isn't: make sure the connection is firm in that
opaque plastic hull. 

-- 
Sami ('75 Westf.)
dakhlia@wuecona.wustl.edu
http://wuecon.wustl.edu/~samid/