I had the same problem with my '72 280E. It went fast blinking all of a sudden but there was no bulb broken. It took a few months before it stopped blinking. This was indeed the relay as Mbenz said. In the relay there is a bi-metal and this bi-metal is just "tired" and stops moving. You have to change the complete relay.
I am not sure where it is on the 450 but on the 280 it was a piece of cake to replace.
My 450sl starts also doing funny with the turn signal. It goes from a little faster to normal and back again, wonder how long it will take till I have to replace it also.