AskSmithy Xtra Statesman misfiring under load

My dual-fuel 5.0-litre VS Statesman has developed a severe issue under load when the engine bogs down, splutters and feels like it's misfiring or not firing on all cylinders. I've had it recently serviced with new spark plugs and high quality ignition leads installed as well. The issue occurs on both LPG and petrol. Could it be the engine is getting too much, or too little fuel? Or could it still be an electrical issue?