Hiya Jerry,
Unlikely to be slow tube degradation then, I'd be looking towards something like a dirty tube lens, dirty top surface of final lens. Also is the spot produced at the first mirror a nice round one or split / miss-shaped? (bit of white card in front of the first mirror to check)
Any idea how many hours you have on the tube? (ball park) it *could* be tube life but they usually die quite slowly over time and not over night although it's possible I guess.....
What speed and feed rate are you using for the acrylic? (is the acrylic from your usual supplier?)
Any unusual noises? (arcing, squealing, loud buzzing) etc?
Has it been moved, kicked

dropped (usual questions)
Do you have plenty of water in the tank?
Just trying to eliminate as many possibles as I can
best wishes
Dave