That was my solution -- but it still leaves you vulnerable. Perhaps it will take a longer hold period, for example, but it will happen again if you aggressively trap in multiball. I ordered some heat sinks from Great Plains Electronics, and I hope that the new transistors coupled with some heat sinks to dissipate heat will prevent the transistors from overheating and failing.

The Stern flipper design is to pump full power through the transistor to fire, and then pulse the current to hold the flipper up once it has fired. But after several minutes, even the pulses seem to be overheating the transistors.

A few posts from my travails here: https://pinside.com/pinball/forum/topic/acdc-flipper-problem-coil-stuck-on

Good luck!