difficult problem with my 1976 650 mercury 65hp

I have a 1976 650 mercury 65hp and have been having some troubles with it. I took it out the other day and it ran great full throttle for about 3 min then it dropped in speed randomly down to i would say half speed with out me touching the throttle. So far we have traced the problem to the ignition somewhere. Here are my list of suspects

1. the coils are firing but one isnt and when we switch it, it with another coil it fires what might cause that?

2 the switch box might not be getting power to one of the coils but idk which one how can i test if the power is getting to the coils we have a multi volt meter?

need help bad. if there is anything else that somebody could think of that might be the problem please let me know