Go here http://caunter.ca/volvo960/. It shows the whole system. There is a set of computer controlled, solonoid operated vacume valves under the dash on the right. It's easily seen by removing the passenger side knee level cover. It controls all the vacume operated actuators. A vacume leak in that system can cause some strange symptoms. If the coolant valve actuator is closing off that would change the heat output but I should think the effect would be delayed, however if an air control actuator is changing position the effect would be immediate. If there is already a leak then only a slight change in vacume such as brake operation or going to idle can cause an actuator to move from it's set position.
Hope this helps. That web site also has a wealth of other information.
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David Hunter