Mk3 Golf - intermittent spark

[SteveG]

Have a '98 2.0L Golf that was running fine a couple of days ago. Go to start it yesterday and it cranked but no start.
No spark from the coil so borrowed a good coil today and connected it up.
Gave it a crank and yay - there's great spark.
Plugged it into the dissy, cranked it and it starts and then dies. Wouldn't restart.
Pulled the coil lead from the dissy, and no spark again.

Kicked a few things, threw the ball for the dog for a while then went back to it.
Now has good spark again so plugged it in and starts/dies
No spark again.

A bit of searching turns up the crank position sensor, ECU or ignition switch as common ignition related issues. Any of those I can definitely rule out from the above symptoms?

Appreciate any suggestions on how to troubleshoot further. I don't have access to anything for reading diagnostic codes etc.

Steve

[Umai Naa!!]

There's a relay that craps out on these, as well.

Pretty sure it's the ECU power supply one.

[SteveG]

Thanks, will look into that too.

At one stage tonight it started, ran happily for 5 mins or so and restarted normally a couple of times. Turned it off, left it for 5mins and no start again. Arrggghhhh!!