its a pretty common problem ..you have to strip down the motor and clean the connections..not a big job
I copy some of the intsructions (e30zone.net...articles section)
Step 5 – Inspecting and cleaning the motor
Now you’ve got the motor out, you can remove the plastic cover and end up with something looking like this:
The main black barrel is the motor itself and the round part on the end is what detects where the wipers should park. It’s this latter part that can cause the problem with stuttering wipers. You’ll see that there is a black plastic cover which is clipped onto this round part, as shown below:
You can prize off these four clips to enable removal of this plastic cover, then you’ll have something looking like this:
A right greasy, dirty mess!! Considering most of these E30’s have gone through nearly 20 wet British winters and 20 dusty summers, with considerable use of the wipers throughout, it’s hardly surprising things get as dirty as this. The electrical wipers on the inside of the plastic cover were also in quite a state, as shown below:
So, time to clean things up. I started with just tissue to completely remove all the old grease, then some light contact cleaner to finish off the job, being careful not to remove any of the grease from the worm-gear, resulting in the following:
Quite a transformation! Next, you have to grease it up again. LM grease will do the job just fine but I managed to salvage a large blob of relatively clean grease from the contacts (see earlier photo) and spread it all over the metal disk and contacts as shown below:
By the way, I believe the notch on the outer edge of this metal disk is what stops the motor with the wipers in the parked position. The two contacts on the plastic cover are for the constant and intermittent/ park settings respectively – one makes constant electrical connection with this plate (on the inner circle) and one circles the outside and breaks contact when it gets to the notch.
Step 6 – Putting it all back together
Basically, you need to do everything from Step 4 backwards as follows:
Clip the plastic cover back on the round section of the wiper motor
Put the overall plastic cover back on the motor
Put the motor back in the compartment
Bolt it to the bracket with the three smaller bolts
Bolt the bracket back to the bulkhead with the three larger bolts (Note that these have elongated holes to allow some lateral adjustment – try to get them back in the original positions to avoid upsetting the mechanism in any way.)
Reconnect the connector (good opportunity to use some contact cleaner!)
Put back the sound-deadening panel
Reconnect the wiper mechanism in the same position as it was taken off
Turn on the ignition and check the wipers park correctly – if not see below
If your wipers don’t park in the correct position, as mine didn’t despite my earlier preventative measures, you just need a bit of trial and error in fitting the wiper mechanism in different positions on the motor shaft until it comes right.
When trying out my wipers in intermittent mode, they were slightly better, but not as much of an improvement as I had expected. However, I considered the electrical wipers within the motor would need time to “bed-in” through the grease, so I ran the windscreen wipers on full speed for a while, then on intermittent for a while and eventually all came good – perfectly smooth operation!! I’ve no idea how much a new motor would cost, but I’m well pleased I saved myself that expense. The total job took me about 2 hours, but if I’d known what I know now about what had to be done and in what order, and didn’t keep stopping to take photos, I reckon it could have been done in about an hour or maybe less. One word of warning though – I ended up with hands cut to shreds due to fighting with the motor through the tiny access holes beneath the air-vent!