I dunno, sounds kinda perhaps like a question you wanna ask in the DOSBox forum instead.
(Moving.)

I've been having the same problem. It seems to occur for me when the game changes resolutions. This only happens when you switch from regular to hi res mode ("toggle screen mode" in the display mode) and when you are already in hi res mode and either a negotation or end of year screen comes up. So it's avoidable by just playing the game at the default resolution. When you do want to use hi res (say for building coasters or just to get a wider view) try pausing the game ('p'), toggle resolution ('r'), then unpause. Seems to work for me. If the mouse ever gets stuck for some reason remember you can save to your most recent savefile with alt-s, then kill dosbox (alt-F9) and reload the game.