I had this same exact problem shortly before blowing a gear in the wiper transmission assembly. My mono wiper still wipes, but in a funky pattern, and with the hydraulic arm in an extended position. (I had to change to a '19 wiper blade to avoid it hitting the panes)
Keeping the hydraulic arm greased is a good first step. You need to get some lubricant down inside the transmission gears that are located in that oval assembly. You can either try to take it apart, or find a crease and squeeze some gear lube inside there and hope it penetrates.
If it goes out on you altogether, you can find a used assembly on the net for $65. I have a shop manual that walks you through the removal/installation that you can have a copy of if it comes to that.
'91 300E 198K