The electrical connector to the back of the gage cluster is making a poor contact. Next time it cuts out, smack the top of the dash with your hand, if they come back on, the connector needs replacing with the updated part. There was a TSB on replacing the connector on '98-'99 TJ's and XJ's.

Check this post from back in April:
http://www.4x4wire.com/forums/showflat.p...rue#Post1243666