Not really any certain thing that causes them to slip over time to my knowledge. When timing them oil on the shaft will cause them to slip or loose time however.

so clean the shaft and inside of the gear really well when timing it? for a little assurance

i would check the screen at the fuel heater if its plugged up it will run like shit
when you time it use brake clean on shaft and gear before its put on

do you have to take the whole heater off or how do you get to that screen??