I have a 1999 volvo v 70 (not turbo) it has 196k miles, however engine has been replaced at 145k with used (rebuilt) due to the timming belt slipping and not catching it on time. Had very little trouble w car. One owner. Recently it was going up to 2000rpms while stopped, but only did that a few times, now the temp gage is running "hot" and the but no lights are on. Engine fan is working and cuts itself off. Car is running and cranks. Have not driven it since it did that yesterday.
It looks like you could have 2 separate issues here. Your high idle I would recommend you have the throttle cleaned and pcv system cleaned out to see if that helps. You may even want to have it scanned for trouble codes to see if anything is pending. If cleaning the throttle does not work then you will need a new throttle, but you will need to go the dealership for this or a shop specializing in Volvos and has access to Volvos diagnostic equipement. For the over heating, how is the coolant level. Is it full? You may want to have the thermostat checked out to make sure it is not stuck closed or stuck open. Even a stuck open thermostat will cause you to overheat, especially when climbing up a hill. If thermostat is ok, usually updating the engine management software will take care of the temp gage issue assuming there is an update for it of coarse. Let me know what happens.