Sure it's not a tilt problem?.  Does the tilt light come on?.

No it doesn't tilt, it just dies mid ball.  Typically if I hit the 10's score relay it kicks off again and will launch the next ball like nothing happened.  Then after it hits a few scoring targets it will reset and become more prevalent the longer the machine is on until it gets to the point where it just dies - nothing will work on the playfield and no flippers.

I have replaced the 10's score reel coil - it was cooked so I don't know if something else somewhere has been damaged as well and becomes sensitive as the machine warms up.

EDIT - I can confirm that the fault is not present when the machine is first turned on and is cold.  I have tested this theory a couple of times now and it works well when it is cold.  As the machine warms up, the fault re-appears and then becomes more prevalent and gets to a point where it won't work.