Threw the belt again after twenty minutes in IMC on the way to the avionics
shop. Destination was VMC, so elected to continue rather than turn around or
land, and the other alternator was handling the load easily.

Avionics guy fixed the radar (took three guys around 30 minutes). Belt
replaced.

Flew 1.5 hours back- no problems. After return I tightened belt again, and
then flew another 1.6 hours doing LNAV/VNAV approaches in IMC (much
smoother than ILS).

Checked belt again, which appeared tight. So, last belt has now gone over
three hours, where the others have all thrown in less than 30 minutes.

What gives- can a new belt stretch so much that it gets thrown that easily,
or should I still suspect that the engine is detuned and has a sticking
counterweight?

The engine shop quoted a cost of over $3,000 as a minimum to change the
counterweight bearings, plus any costs associated with finding something
wrong with the cylinders.