Arfa Daily wrote:

In what way had the original Tx failed ? Might have some relevance to the
problem, which although may be heat-related, might not be a 'heat' problem -
if you see what I mean ... For instance, it may be that one of the supply
rails is dropping due to a monolithic regulator IC going into heat foldback,
but that could be because something on the end of it is drawing too much
current.

Or the screw holding it to its heatsink may have come loose.

Basically it needs proper fault finding ! Monitoring the supply rail(s) is the
obvious starting place.

Graham