Quality and Workings of Rotary Encoders

I recently bought two boards with four (4) rotary encoders on each board. There's a USB connector and when I connect it, my OS (Win 7) recognizes it as a "rotary encoder device" and it programs just as one might expect. However....

Sometimes, when I use it, it will move the altitude or speed either by really big numbers or really small numbers. Sometimes, for example, the altitude will vary by 1,000 feet instead of 100 feet per click. And the one I'm using for speed hold/control often goes up/down in single digits instead of by 5 or 10. It takes a long time to get from 0 to 250 if it's only going one at a time!

Does this reflect the quality of the rotary encoder, is it a function (bug) of FSX or Windows? or what?

I have one board set up for HDG, ALT, OBS and SPD. I'm reluctant to set up the other board - still need BARO, V/S and maybe something else - until I figure out why what I have won't stay stable.

I can well remember what company I bought them from, but I don't want to pick on them. It may very likely be my expectation that's the problem, not the manufacturer.

Art - N4PJ
Leesburg, FL