Heater Valve / Matrix Replacement

[joker]

Have intermittent heat from my heater. Literally blows hot one minute and cold the next but more often cold. Engine is running at correct temp. Matrix does not appear to be blocked but not sure. Think that valve is at fault but no leak inside car.

I'm going to try and get a replacement valve. Would a valve from any E30 fit or specific to 318is? Same question for matrix too. If matrix is not leaking, seems like a big job to replace. What is best way to clear? Have heard of plumbers stuff called "One-Shot". Its an acid that clears pipework but does not damage metal or plastic pipe. That be ok???

Thanks