Toyota Lexcen Engine Problems

No, using an injector cleaner would only help if you were running on petrol, it would have no effect at all if you were running on gas. I would suspect that the engine is tired and in need of a freshen up, particularly if it's been running on gas for some time. Installing new plugs and leads would help, but I would suggest you have the compression checked. That would tell you if the valves are sticking, or even leaking.