Hi Dave, intermittent problems are sometimes the hardest to find and sort! From your description of the missing and backfiring, I would have a look at the electrics first. Check all the connectors, plug leads, coil lead, rotor arm, Cam Pos. sensor etc. Also the fuel injector trigger wires in case one is loose. If you've got an earlier 850 I'd pull the error codes - it may be that one has been set without triggering the engine check light. Could it be a sticky Idle Control Air Valve? A small amount of emulsified gunk on the top of the engine dipstick is normal as this part is in the cold air flow and condenses any water vapour given off the oil.