Had a problem similar two years ago. Stalled out under low speed lug the first time out. Started 30 minutes later. Happened again at the end of the spring. Ran good in between and thru the fall.See the posting "Smoke and mirrors-well mostly smoke" about this time last yearIJ if still available. Upshot of the whole deal is: 1) there is a bushing on the timing advance in the fuel pump that was seized even though pump was checked and tested good 3 times--there is an update for this problem; 2)pulled the engine down and found 2 broken rings, all ring grooves badly worn, 1 liner moving vertically enough to let small amounts of coolant into the cylinder causing pitting of the head and valves, 2 cracked valve seats. Was able to save the head. Net result was on inframe overhaul. Expensive, Yeah, but probably saved the engine for the next year. This was a 426 @ 5500 hours. Pull the head and check inside or at least do a compression test. BTW, most of the smoke is gone. Good luck on this one. Rob.