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?

I have several rb133c with 5.21.periodically cpu goes 100 % and tool profile shows that flash uses all cpu.
Question:dead rb flash?

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