similar problem here on a project i have. been told it could be something to do with temp sensor under the manifold.
mine is fine when cold too untill warm it just hunts up and down and almost cuts out.
mines heats up just as quick new rad and fresh coolant in october maybe helping a little.
BUT i have noticed a slight drip of coolant on the top of the header tank maybe the alloy pipes havea crack or something
sounds like dead maf mate.
had the same on friday done ecu read error and code 23.
reset and left running for 5 mins and was fine,turned off and then started it and bam same again.
new maf should be on its way
sorry to hear i am on the marlet too for a 2nd hand engine so i cna have my current slappy engine puleld and forged.
just hope for some better luck wit ha 2nd hand that yourself