Had similar problem on my Mk1 and never really sussed it out, went away after I cleaned up the ECU and earths.
When you say it wont start - I'm guessing the starter motor turns? Are you getting spark? Does the rev counter move when turning it over (assuming it does when the car is fine, mine does)
Could be the crank sensor but there's plenty of other things it could be. Check for damp in the dizzy cap, incase the heat causes condensation or anything silly like that.