2000 E39 Heat inop

I had this exact same problem last year. The heat won't work properly. It comes out cold and maybe 15 minutes later blistering hot, then back
to cold again.
Last year my local mechanic replaced a "mono valve" and it fixed the problem. One year later back to the same problem.
Incedentally, now my enigne is starting to overheat. The temp climbs to the red for a very short time and comes back down again. Thermostat? This will be the fourth replacement in the 3.5 years I owned this car! what gives??????
Any comments welcome.
PS, I'm not good enough to replace the thermostat, but how difficult is it to replace this mono valve?
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