If it's turning over ok then the next thing to check is that you have a spark and that you have fuel pressure.

My S1 had a similar fault that only appeared when it was warm, (but not all the time) and that was the plug on top of the dizzy, new dizzy and the problem was sorted.

Next time it wont restart, try cranking it over a few times and then whip out one of the spark plugs to see if the tip is damp with fuel, if this is ok then pop the plug back into th elead and get somone to spin the key and see if there is a spark, you will need to earth the plug against the engine for this to work.

Make sure you dont hold the plug or lead, as although it is an easier way to tell if it's sparking, I don't think it's great for your health.

As I said earlier though, you may find that the fault is with the dizzy breaking down when warm.

Your in for a fun weekend !!

Good luck

Rich