"Take the oil cap off when its running and see if it makes any difference..
If it runs lumpy there is a prob...."
But you say it cuts out, so I think u got bigger prob...

Try this - with car ideling quickly take oil cap off and put your hand over the filler hole,
if there is a fair bit off air pressure there that would indicate pressure is getting into the crankcase...........
cause of that could be -
worn bore / piston, problem piston ring or cracked piston.....
Do a compression test first to assess condition of engine / eliminate that possibility ....
Halfords sell compression testers. well worth buying

You say it just suddenly cut out....
Was u giving it big boost prior to it cutting out ???