Had/have a similar problem, Car (1996 EL 6cyl S/wagon -tickford Lpg) -was Mid 2011, running rough,Replaced spark plugs(with ngk bpr5ey-11),plug leads(Gasmaster LPG leads),Dizzy cap and rotor (OEM parts),3 old plugs had bad corrosion on the ends of the threads. Runs better.

10 months-18000K later( Now) same problems,getting worse, Starting on 4 1/2 cylinders,Rough for first 15 seconds then sluggish until warm,Lack of power.
just checked Plugs again, 3 badly rusted away on thread ends (again) .Replace with good spares, Running well again.

Leaking head gasket?, no real loss of coolant, 330400k on clock. haven't done H/gasket. brought at 265000 K

Ideas?

Booked in for a tune and trans' service- Monday week.