Ren:

Don't do this if you feel it is too dangerous but I had the same exact problem and "unclogged" my radiator by happenstance. I opened up the radiator cap while engine was hot (to first detent for a moment, then all the way off when some pressure released -- use large towel if you do to not get burned) and the backflow of pressure/water had released a small blue strip of rubber that was the culprit.

Is this your original radiator? A radiator shop can unclog it effectively for around $35, unless it is really bad inside there. They symptoms you described, however, suggest that something got lodged in there just recently. (not a slow buildup of scale or grease), so I'd try the fix first before dishing out $220 for a new one.

Good luck with whatever you decide...

~Paul

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'91 300E, 210K miles