Periodic front brake drag problem

[swheele2]

My brakes work fine most of the time, but periodically the front will start dragging significantly (car feels very sluggish, engine temp climbs to about 100 C). Eventually the problem will "fix itself" and it will start driving normally again (this happened a few weeks ago 50 miles into a 100 mile drive. After a few miles of dragging brakes they loosened back up and worked fine the rest of the drive).

I installed new (rebuilt, ATE) calipers and new flexible brake hoses. The brake fluid has been flushed multiple times at this point (I also replaced the rear calipers) and the fluid is perfectly clean/clear.

Since both front wheels are dragging equally, I assume it's not a problem with a rebuilt caliper. The previous owner had a new master cylinder installed May, 2008, so the MC should be fine, but I'm not sure what else could be the problem. Could it be that the "new" MC is bad?

Any suggestions? All of the previous discussions of brake dragging I can find are not intermittent like this (or at least that wasn't the impression I got).

1984 300D Turbo (200K miles)

Steven