I had a similar problem with my 260E. I replaced the thermostat and the problem persisted. I thought it was a problem with the thermostat and I drilled a small weep hole in it to test it and it did cool the car down a lot. That indicated that the thermostat was only opening part way.
Got a replacement for the deffective thermostat and the replacement works fine.
Good luck with your repairs!
1987 260E Charcoal Grey 157,000 Miles
1987 260E Forest Green 120,000 Miles