Try enabling sketcher_dimension_autolock and sketcher_lock_modified_dims if they're not already enabled. I can't remember having issues with sketch instability since I enabled them.
The default normal blue dimensions will change if you drag them, or make changes to other dimensions. Be warned though: with dimensions autolocking, if you define other dimensions that make your current ones invalid, Creo will just ignore you.
If you haven't already, set visible_message_lines to at least four. Creo often gives multiple-line errors, but the default display of 2 actually only shows one line. If you can only see the one line, it changes so fast you won't notice the difference if you're trying something multiple times and Creo keeps generating the same 2 or three line error. If you can see two or three message lines, you'll see the entire error message and you'll be able to see the line movement changing, whihc gives you a better idea of what you're doing wrong.
