...will not shift into overdrive. At times, it will shift into and out of overdrive several times, but will not stay in OD. Any ideas???
Yes, I serviced the auto trans, hoping that it would solve the problem but it did not help.

Have you changed the filter and transmission fluid recently. This sounds like low fluid level or foaming.

Auto trans? 300SL's come in both auto and manual
sounds like the vacuum governors on the side of the trans needs adjustment