At least a couple times a week I start my 81K miles engine and it only runs three cylinders but if I rev the rpms up the fourth kicks in. The other symptom that goes along with this is that when I do rev it up it takes a long time for the rpms to come back down. All I have done to try to remedy this is to put a bottle of Lucas fuel treatment in a full tank but it did not help. I know next to nothing about engines but I am thinking it might be my fuel injectors. Please help if you have any knowledge in this area.