Did you do what is known as full/complete firmware reload? (Requires pressing Firm 2 + Firm 3 + Firm 4 within 2-10 seconds after reboot).

Try that if you have not done so. That will hopefully correct the issue but I cannot say that 100% as I have to wonder if this was not caused by programming added to the unit.

With regard to "suddenly one day", what changed in your configuration that one day?

Are you using System.delay() in ProntoScript? Are you using onMove() for buttons in ProntoScript? Are you using large images for which you specify that image be resized on a widget? Are you using dynamic widgets that you are not properly destroying? Are you doing serial.recieve() or serial.match() in a loop without using scheduleAfter()?

These are just some of the items off the top of my head that could make the unit operate "sluggishly".