Sudden changes in program behavior usually make me suspicious of various issues outside of the software itself.
Some of those might be…
• a plugin incompatibility.
• background processes from other programs.
• in the case of multitasking—switching back and forth between running programs can occasionally create issues with how the OS manages the focus back onto the ‘active’ program.
• alternatively coming out of sleep mode can sometimes cause performance anomalies.
. . .[these last 2 are usually fixed by restarting the software, or rebooting the computer].
If the model has been corrupted, which is a fine issue to consider, are you able to repeat the problem?
I trust that other .sib files are working perfectly fine??
What happens if you bring in either or both of the components into a new drawing? does it start to exhibit the same behavior as well??
Even bigger questions: Is this the same SketchUp program you’ve been using for 5 years?
What version are we talking about here?, and on what OS is it currently running? How much memory, and what is your graphic card make/model?
Have you recently installed new software?.. SU extensions, OR other programs, (including updates, (automatic or manual )).
Did you look into changing the ‘Hardware Acceleration’, or the ‘use Fast Feedback’ settings within SketchUp.
What are the statistics on your model info (re: face and edge counts), is it a huge model?, or is it relatively small??
Don’t mean to challenge you on your assertion that it’s not a mouse or button problem, but if we don’t know what the problem is yet. . . how do you know this isn’t the case? […speaking from a trouble shooting method which keeps everything on the table until eliminated.]
It would also be nice to post up the model. a model not displaying well on your machine suggests other things IF it displays successfully on another system.