my mobile dies

Gyus my mobile has very strange behavior . restarts without reasons, imost times cannot connect to wifi and other strange things . it got to a specialist and he said that probably is the battery . One else said it is the cpu and that its going to die .
I have it pure stock. no root no nothing. i wipe everything and reflash the latest rom but i have the same problems
can somebody help me?
thanks in advance loizos