Funny things happen when car is hot?
 

The Engine in my 1985 380 SL has been shutting down after reaching operating temp. This happens at idle speeds. At the exact same time the horn will not work. After cooling down a bit the horn will start working again. After the engine shuts down it recranks and runs with no problem. Any Ideas? Could it be the OPV ? Why would it only affect the horn? Thanks

The only relationship the horns and the ignition have is that fuse 8 (which only powers the horn) gets its power from the same ignition switch power leg.

This might mean that the ignition switch is bad. It also might just be coincidence. Check the power on fuse 8 and terminal 15 on the coil. They both should be hot and both are powered off term 15 at the ignition switch.