I've had the game freeze several time, and it always seem to happen when the enemy breaks through the shields.

I experience the same thing myself. Doesn't seem to be any pattern, though, it just occurs now and again. Usually when I'm at the last point of a long travel......

I've had a few freezes. All of them seem to be when I probably would have been killed by a collision, either with an enemy ship or asteroid after losing my forward shields. Hasn't bothered me much as I was gonna die anyway. Just a little more involved to kill dosbox and reload.

Yeah, it's fortunately not at the same times and places, it's just annoying when it happens, especially in the end of a long trip.

Unless DOSBox itself has frozen you can switch to windowed mode (Alt+Enter) and close it like any other window.

That is what I meant by a little more involved. Not just a plain reload like when you blow up. Requires an Alt+enter or Alt+tab, a click to close the program, a double click to relaunch, and an ESC to skip intro screen. More than a simple reload but sure beats having to press the reset button and rebooting DOS like I had to do in ye olden days.