I have several rb133c with 5.21.periodically cpu goes 100 % and tool profile shows that flash uses all cpu.
Question:dead rb flash?
Few things you can do here..
Disable V6 package
Check that there is no graphing enabled (/tool graphing)
Check that webproxy isn’t using the flash as its primary store (even if you aren’t using it) (/system store)
We still have about 200 133’s in production using these methods 
i have 5.21 installed, no webroxy or graphing
Yeah, but if you check under store what does it say?
They are just running out of resources, If you want to do anything apart from very simple functions its best to leave them on 2.9.51