If you have the same problem on both fuels it has to be something common to both and the ignition side is the obvious way to go.Coil packs normally flag up an OBDII code so I would go for the plugs 1st, not sure if the plugs are suitable for dual fuel but iridiums are the nomal way to go,not cheap but last almost forever.
Another place to check and clean if needed is the throttle body which can cause poor idling/stalling issues.
I owned a 1999 GS300 for s short while and loved it but not as much as my LS400 so it had to go,still owned by a local guy who repaired my accident damaged LS.
Talking local I see you are in Derbyshire,anywhere near Alfreton?