I have a 1988 500sl and a 1988 420sl that does the same thing. I have replaced torn monovalve inserts on both but hasn't fixed this issue. They had heat full time to start with and the monovalve inserts have fixed that. The system checks ok everywhere else and I suspect plugged heater cores. Tried to flush both and maybe a little improvement. I have unplugged the aux water pump on the sl to slow the water flow and it helps but its not fixed. Both customers have declined repairs so I cant say 100% if that is the problem. Curious to know if anybody has had a heater core replaced to fix this problem.