We just got a used 2000 Outback last week and today we started to overheat. It only got hot when we accelerated but stayed stable when not trying to push it. When we looked at the overflow res. it was boiling. We did not let it get to the red line but it did get pretty hot. There was no white smoke from the exhaust and no water in the oil. What could be the problem and what can I do to fix it?