Hello. I am seeing the same problem, but I was using dosbox 0.74 with no issues for about a week when it suddenly began happening. I attached an image of the issue. Here is a quick description of how it started happening for me:
This instance, my mouse and keyboard were coming through the program synergy (it shares mouse/keyboard over multiple systems via tcp connection). I started dosbox, mounted c:\dosprog and then started DOOM and my mouse input malfunctioned in a way I had never seen. Every mouse input made the doom guy move in the same direction. This was the first time I played using this Synergy setup and the problem with the mouse was there as soon as I started DOOM. I assumed that using Synergy was the problem. I stopped the synergy client (the server providing the mouse/keyboard was on another machine), and I plugged in a local keyboard mouse with dosbox still running. After about 5 seconds (literally) the video went crazy (hard to describe) and then returned and the mouse input was still not working correctly.
I closed dosbox and re-opened thinking it would fix the issue. I tried to mount my c:\dosprog directory and that is when I noticed the 'm' just kept repeating. Now EVERY time I run dosbox, the first keystroke I make is repeated forever, until I put another application in focus. Then the keystroke repetition stops, but when I return to dosbox it will no longer accept any more keystrokes for awhile. When it does accept them again, it does the same infinite repetition. It is basically frozen, but the cursor is still flashing. For example, if I start dosbox and then start to type 'mount ....', the 'm' will start repeating (as if I have it held down) and then it behaves as I described above.
I restarted several times. I installed dosbox 0.63 and the same issue occurred. I moved the dosbox folder to a thumb drive and still it will not work. I deleted my c:\users\*****\appdata\local\dosbox directory and re-installed multiple times. I tried all the different CPU settings after reading this post and so far nothing has stopped the repeating first keystroke. I toggled hyper threading in the bios and ran with hyper threading off and then on again. Same problem in every situation.
I do not think I have a key mapping file, so I have done nothing with that. I am wondering if a registry setting became corrupted or something?
I am running an Intel Core i7-2820QM CPU @ 2.3GHz and 2.3GHz (both cores). 8 MB RAM, 64-bit Windows 7. I did install Windows XP mode in my windows 7 (basically a WinXP virtual machine) and dosbox ran correctly inside the Windows XP VM, but running a VM inside of a VM was not a great experience. Can't really play DOOM with the mouse and it is choppy as all heck.
Again, it ran great until I tried to hot-swap my mouse and keyboard with dosbox running and now multiple versions of dosbox all have the same problem.
Please let me know what else I should try. Thank you in advance for the assistance. I was really looking forward to playing all my old games on my upcoming 10 hour flight to Europe.
p.s sorry for the length, just trying to be thorough.
Regards,
Brent