gap closing on spark plug

[metalforsale]

The spark plug gap on the rear cylinder of my 2000 Kawasaki 900 STX keeps closing. The manual calls for a gap of .028 to .032. I have been
setting the gap at .028. After running for a couple of hours the gap closes in. It has done this the last three times I have taken it out. I checked it the last time out and the gap closed to .018. I am using the NGK plugs the manual recommends and have used three different plugs. Thanks for your help