Engine cutting out when it gets up to temp

[Rick]

Whilst running the truggy this weekend we had problems completing a heat.

The engine starts OK and ticks over, there is blue smoke from the exhaust and it accelerates OK.

The problem is about 3 mins into the race it cuts out, we retrieve it and it restarts OK and then lasts till the end of the 5 min heat. During the 10 min final however it cut out again after about another 3 minutes.

At the end of the heat we check the temp at around 120 degC.

Engine - Novarossi N21B
Plug - Novarossi C6S
Fuel - Byrons Gen2 25%

NB
It ran Ok last time out, the only change being fuel. We ran Byrons 25% before (just changed to Gen2).

Any ideas?