Rob,
Some epoxies are tricky, here's a few things to check:
1.) Temperature rating of the epoxy - The water Temp shouldn't be that cold on Champlain but it's something to check. How cold is it getting at night up there?
2.) Some epoxies have incredible "tensile" strenth but will break with very little "shear" force. I don't know anything about flashers, but if the transducer is mounted on the bottom of the hull and your driving 60mph, that's going to apply some shear force to the side of the transducer.
3.) If it's a 2 part epoxy, make sure you are mixing it properly.
4.) Some epoxies have a "best when used by" date on them. Make sure it's new epoxy and not something that's been kickn' around the garage for a few years.
May not be the answer, but just a few things to think about.
Good luck,
Marke