In my case, the contacts were worn (frictional force chewed up the material a bit) so the rotor was not making good contacts with those on the cap, especially when hot (the cap must have been expanding a bit more than the rotor so the rotor cannot make good contacts.

Once I took the cap out and the wear was quite obvious.

Your case might be different but it is possible if you did not remove the rotor to check the inside.