Problem solved!
Replaced the distributor cap and rotor, then let it idle for 10 minutes; no dramas.
Took it for a run out to the Launceston airport roundabout (110 km/h section) and return, no dramas there either.
Thanks @thetrough for the tip. What threw me was how it was still getting spark and running fine when cold, but it must not have been enough spark to make it run when warm/hot.
Thanks all for replies.