Mine started doing that, but it started on its own. I was scheduled for a coolant change and while I was at it changed the t-stat housing from plastic to metal and while I was at it change the t-stat. I turned out the t-stat had physically broken. Now everything is ducky.

BTW I didn't burb the system but everything works fine, maybe just thinking about it was enough.

Does your's have a plastic housing?

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5 speed '91 190E 2.6 320,000 mi. (new car, fast, smooth as silk six, couldn't find any more Peugeots)
5 speed '85 Peugeot 505 2.5l Turbo Diesel 266,000 mi. (old car, fast for a diesel, had 2 others)
5 speed '01 Jetta V6 (new wifes car, pretty quick)
5 speed '85 Peugeot 505 2.2l Turbo Gas 197,000 mi. (wifes car, faster, sadly gone just short of 200k )
5 speed '83 Yamaha 750 Maxim 14,000 mi. (fastest)
0 speed 4' x 8' 1800 lb Harbor Freight utility trailer (only as fast as what's pulling it)