30coupe1

I have an 89 mustang supercharged, it was tuned and driven around awhile and then lost a head gasket. I changed the head gasket and when i got everthing back together i was not sure if i had the timing 100% right so i took it back to the tuner. When he was trying to time it, the car didnt want to hold time. He set it and then it would change again. He said it could be the ignition module. Does anyone have any ideas of what could be the problem?

Thanks for any info