Nick, first off you not running the H.O Jeep V8 your is normal V8,
Next - have you been using your flash lube as I mention when you were at mine, if you over egg the oil additive it may run a bit lumpy, it is meant to be one drop on fluid for every 6 seconds running at idle.
Also when I had this same problem, and it was not long after I had mine converted to LPG mine had a bloody good service and LPG specific spark plugs fitted, they were not cheap but made a massive difference.
My initial guess would be change the plugs and set up the Flashlube properly.