There are a couple of other threads on this topic but briefly, I suggest the following.
1) Disconnect the wiper arm linkage from the rear of the motor, switch it on and check it has reasonable slow and fast speeds.
YES/NO?
2) Now check the mechanical linkage, you should be able to operate the wipers by rotating the motor arm by hand.
(Lift the arms off the screen or remove them to eliminate drag on the glass.)
YES/NO?
NO to 1) means motor is tired.
NO to 2) means that the pivots and linkages need a damn good lubing!
Fixing the pivots is relatively easy, just be patient in getting them apart if they are dry and corroded. Lots of CRC-556 and gentle pressure over several days and they should eventually come apart. There is or should be a wuzzy little circlip on the end of the shaft, remove and ditch it. Replace it with a decent external circlip from CBC.
Wire brush the shafts, thoroughly clean the tubes, lube it all with PH Zinc Oxide grease, check for smoooth rotation and finally fit new weather boots to the shafts