I've checked out the posts on this forum and it seems that several people have the problem of the left and right vents pumping out hot air while the center pumps out cold. I've seen several posts as to the fix, the most likely culprit being the diaphragm units behind the dash. Mine has one other symptom... after I've been running for about an hour and a half (almost to the minute), all of a sudden the heat shuts down and all vents pour out cold air. I can then crank the thermostat all the way up to get heat flowing again and then turn it back to 70 degrees.

Does this help narrow the problem down any? What would be time sensitive in the system?

The car is a 1991 190e 260.

Thanks