Recently, my '80 245 has been running at very high temperatures. It's come awful close to the red more then a few times, and I'm starting to get nervous.

Before this started, something similar has been happening. When I would turn the car off, the temp would start rising. If the car was running, the temp would drop back to N, but it would start rising again after I shut it off. I raised the hood (with the car off) and I could hear a kind of whistling/hissing noise coming from the coolant tank cap. The coolant level was still within acceptable amounts, so I didn't think much of it.

I know the temp gauges in some of the later year 240s were junk, but this is an '80, and it's never been faulty before. I also have plenty of reason to believe the temp is running high, (for example, when it rains, water immediately evaporates off the hood, and continues to do so for quite a while after the car is off. It never did that before all this.)

Any ideas?