It seems you have been unknowingly finding all sorts of problems with the FMS, eh?  ;)

Something similar has happened to me before. But it was a flight to La Guardia (KLGA) I think. I will try to fly it and post if I found the same problem later. (Where the aircraft starts on an un-commanded climb "at 200 fpm to God knows where".)

What ever caused it is rare and almost never happens, at least.