Engine back fires when at normal temperature

Engine : 1.3 K3-VE
Extol van

engine starts okay runs okay, cold until it reaches normal running temperature then when you accelerate it back fires and has no power.
replaced the engine coolant temperature as it had a fault stored for it. No faults get entered in the engine control unit while the fault happens. Checked the valve clearances all okay and replaced the spark plugs. Catalytic converter will glow red. Any ideas?