I've seen this caused by bad convergence ICs. If you unplug the convergence
board I believe you don't get video, but it won't cycle.

Incedentally the coolant is very hard to change in those. I've done it and
gotten real good results, but it is a huge PITA.

If you can get them, STK4191 is the upgrade from the STK4121s the set came
with.

JURB