Start with the tacho eh? How have you connected the tacho signal input wire to the bike?
The ebay tacho doesn't seem to have the degree of filtering on the input that more expensive units have. This means that it is susceptible to additional electrical noise that might be on the wire that you hook it up to - the wire from the ignition control unit to the coil primary for example. It doesn't take much of a circuit to filter this out, if you're up for it.
The other (much more sensible) way is to send a bit of cash in the way of one of our brainy forum members who has designed and built bespoke circuit boards, just to do this sort of thing, and decode the gear position indicator too!
Edit: the advantage with the bespoke board is, I guess, that it conditions all the K100 signals etc. so's you can use pretty much any aftermarket tacho/multifunction display etc. The hard work is all done for you. Search for 'TGPI Board' on this, fantastic forum.
Edit2: If you want to do it the painful, dumbass diy way, post 75 on the 'K75 Madass' thread will give you an idea of the trauma involved.