https://www.cip1.ca/ProductDetails.asp? ... 13-16-9290
I 1st tried to use a drill on the sender and I was getting a very steady reading, weird! On further inspection when I looped the cable back in the spindle and turned the wheel be hand, I noticed the cable would 'bounce'. It would stick then turn rapidly and stick again while the wheel was turning at constant speed.
Long story short, the magnetic pickup sender was good but the cable was bad. I replaced the cable and we now have a very stable speed reading. If you're using one of these sender units for VDO or Speedhut and you're getting a bouncing speed reading, check the actual cable.
Thought I'd share.