not familiar with the antique you mention but YZ250 is on the right track by the sounds of it
poor circulation is overheating the boiler either switching the overheat stat or over run stat
could be the pump,or blocked heat exchanger or pipework
is it closed loop or header tank fill ?
sounds like a good flush through and check the pump would not go amiss
obviously , care should be taken not to dump water all over the electricary gubbins (so isolate)
and don't dick with gas settings or connections
IF it is a gas related problem,like modulating gas valve bummed ,you should pay and have a "gas safe " engineer to rape you
even though you flushed it in january, you can't rule out sediment of a 25 year old system being flushed from a rad and blocking the boiler ,say when some inpatient **** whacks the boiler up on a Friday after being away