Am 21.08.2016 um 13:19 schrieb Aaron Jackson:
I am guessing that the 74LS243 is just soldered on the board somewhere? Perhaps if I replace it my machine will become much more stable?
Before doing that (with a possibility of damaging/bricking your precious machine), you could try to actively cool the IC and verify that it is really the cause of the problem.
There are special heat sinks for DIP or SOIC parts, or you could use a small fan, or even coolant spray (careful, not too wet! :)), depending on how frequent/repeatable you see the stray interrupts.
Felix