Fast idle thermovalve

That would depend on what is wrong.

If it has worked itself loose, replacing the Oring and readjusting would do but calibrating it might require trial and error.

If the coolant passages are clogged, you need to clear them.

If the thermovalve is bad, it has to be replaced.

From symptoms it seems the problem might not be due to thermovalve alone, check the idle adjust screw.