Yesterday morning my 500SL acted like it was between gears, engine revved fine, but it had no drive. I shifted down to 3rd and then back to Drive and the problem went away for the rest of my commute. Last night on the way home, it happened again but nothing I could do would make it go into 4th gear. I checked the fluid level and it is right on the mark, the fluid is clean and has no abnormal odors. No leaks from anywhere. I have no clues. Anyone have a starting point for troubleshooting?

Thanks in advance.