Well I fought this unstable/is it correct? tach problem for awhile and got really tired of it with stuff that was sensitive to where the sun was or shadows interfering with readings.

Then Fromeco started building TNC tachs again and that solved my problem... [8D]

http://www.fromeco.org/shop/atnc%20tach.htm

You can buy them direct, or there are several other vendors that stock them. They are a little more money but pretty much eliminate the problems you've noted. Those tachs were very well thought of around here from what I was able to find, but had gone out of production until Fromeco got the rights to build them again. Try it, you'll like it!