1987 300sdl - 95,000 miles. I've read many of the no heat threads having to do with hitting certain speed / rpm and haven't found quite this problem. Mine has heat until I reach 2000-2200 rpm and then it goes cold. Unlike many I've read, just dropping below that rpm range will not start the heat back up. I must turn the key off and on - staying below that rpm range. As soon as I go above that range, heat goes off again.

I ran like this for a couple of months. This week I pulled the mono valve cartridge and replaced it with a new one. I was happy to see a tear in the diaphragm of the old cartridge and was expecting success.

Nope.

Still does the exact same thing. I pulled the new valve to make sure I hadn't torn it when installing - it was fine.

I pulled the plug on top of the mono valve and drove the car, this doesn't make any difference - still goes cold above that rpm range. Still must turn the key off and then on.

Used a test light on the plug and it lights up when the key is on, so I replaced the plug, drove again and...

Nope.

Any suggestions?