Running hot, 1984 500SEL

[pmckechnie]

I have a strange problem I would like to get some ideas on. My 84 500SEL runs about 188 to 192 F in normal driving. It may go to 200 in hot weather and sitting in traffic. Now the strange problem. Car sits overnight, outside temp 40 to 50 deg F. Start, drive a mile or two then get on an interstate or highway going 50 to 65 mph. Temp will go to somewhere around 215 to 220 deg and with in 2 or 3 seconds will start back down and be 185 or so with in 10 seconds. Doesn't do it every time. doesn't do it if the speed is under 50 mph for the first 15 min or so. I changed the thermostat yesterday and it did it this morning. The old thermostat only had about 10K miles and 18 months on it. New thermostat is a Behr, don't know about old one.
I don't think this is a serious problem, but would like to fix it. This has only happened 4 or 5 times in the last 8 months.
Any ideas?

Paul