Guys, thanks for stepping in and making suggestions during my xPages struggles.
2 Things I did that seems to have stabilized my xPages environment:
-Uninstalled and reinstalled 8.5.1 and Fixpack 1 (Thanks Dec!) - My install has been copied over since R5 so it was probably due for a little crust removal. Seems to have worked out nicely. Faster and much more reliable - though not crash free....
Recreated the xPage and re-embedded all controls (Thanks Stephan!) - This seems to have fixed the worst of the weird error messages, though I still do get the usual weird error messages that everyone gets when working with xPages :).
After all this I think I'll write up a getting started xPages checklist - where to go, what to do, what not to do....
First Tip: For large xPages with multiple custom controls, create each custom control in it's own xPage during testing and debugging, then combine them when you know they work the way you want. Saves time in many, many ways.