John,
I am guessing that the 50MHz oscillator is the type in a DIL metal package with just four external pins. The output is driving four Schottky high speed TTL gates which is quite a load (each gate about four times as much load as the usual 74LS devices), but no doubt within the spec of the oscillator package when new. It is possible that one or other 74S112 is moving out of spec and presenting an even larger load which is causing the oscillator to heatup and fail. You can get equivalent switching speeds with modern 74HCT devices and being CMOS they have very small loading on the input side. You may be able to extend the life of the oscillator can by replacing the two 74S112s with two 74HCT112s, if the ICs are socketed that would be a lot less hassle. You can get the 74HCT112s on eBay in the UK and probably many other suppliers as well.
Try pulling the two 74S112s and see if the oscillator is then stable.
Roger
