Stuttering instruments in virtual cockpits

In the virtual cockpits of several, but not all, of my downloaded aircraft, a strange fault has developed. The instruments update their readings every half second or so in a series of jerks, whereas the flying itself continues as smoothly as before at my selected framerate of 20fps. 2D cockpit instruments seem unaffected. I have VFR photo scenery and Active Camera installed and have had no problems for many months. What have I done?