Since this is NOT an intermittent problem, you would be money ahead to find a good, honest shop in your area that can troubleshoot and repair this for you. Replacing parts by guesswork is an expensive proposition.
I have done some shotgun troubleshooting on the same problem with my 300E, but mine is VERY intermittent so there's no since taking it to someone who knows the system well because it's guaranteed that it will behave itself perfectly while in their hands.
The best I've been able to do is get help here based on symptoms and no testing by the technician. Even the very sharpest troubleshooters cannot help if the problem is not actually there when testing.
You did not put in your signature where you are located, so we don't know if you are near one of the known good shops or not.