I have never seen that problem, but make SURE you have a clean autoexec and config.sys file in order for it to run properly No emm etc.
There can't be ANY drivers loaded for TCNC to run properly. IF you are running V4.x, you can before typing TCNC.exe, load mouse.com so you will have mouse support.
In my case, I do it from a bat file:
mouse.com (if it is not in the root directory, you will have to give it a proper path)
CD/turbo (or what ever directory TCNC is located)
That should do it.