I bought a 90s 75 Twinspark with 6 months work of NCT/MOT off a mate for 500 euro about 10 years ago - primarily to have a bit of fun on track. Got one track day out of it - which was fun when it was going, but It had two distinct operation modes - completely fine, and completely dead. And would switch from one to t'other with no warning. Died on me out on track twice in the one day. After the 2nd time I didn't fancy running the risk of being clubbed senseless at the back of the paddock with tyre irons by the other non-too-happy drivers who were in the sessions which were red-flagged, so that was the end of that.
Discovered (eventually) it was a bad connection in one of the electrical relays - but by that time it was too late, the damn thing had oxidised itself into oblivion on the driveway.
About 3-4 years after I bought it, two passing gentlemen from the Hi-Ace driving fraternity offered my wife 100 quid for it one day when I was at work - and the deal was done. Gone by the time I got home. Biggest regret I had was not getting a chance to whip the steering wheel off it before it disappeared:
I still regret that loss