I've been doing some work with our 740 recently, but now the V70 is having problems. The heat is absolutely terrible, and the defroster makes window fogging worse. Here are the symptoms:

-Heat (even set at 80F) is very poor, unless car is at a dead stop and idling. If it has been idling for a long period of time, then the heat is so-so.
-Temperature gauge on the dash gets up to operating temp at idle and at speeds up to 30 MPH. After that (such as at 65 MPH highway driving) the temp. gauge goes all the way down to zero.
-Defroster makes window fogging much worse. It is blowing condensation onto the windshield.

I searched through the archives and found that the last symptom is usually related to a heater core leak. The carpet is not wet, and there is no smell of coolant from the vents. There is a small smell of coolant in the engine compartment. Since I have not been monitoring the coolant level (it's not my car) I can't be sure if the coolant level has been going down. It is about halfway between the min and max marks right now, and it is distinctly green. Is the green coolant the correct stuff?

On my Mercedes, a stuck heater core monovalve was the cause of poor heat, but that was at all speeds and idles. It was stuck because of the use of incorrect coolant. Is there any such valve on the V70? And, more importantly, can I fix it myself?
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'89 740 Turbo, '99 V70 GLT