VTSSC40N problemes


I have a problem with a VTSSC40N solder station. After switching it on the temperature of the unit goes quick up but keeps on moving form 350 to 450 and back. Never constant. The real temperature is not reliable.
I replaced the iron, but the new one has the same problem.
Inspecting the inside did not give me a clue.

Any idea what the problem can be?