Could be simple... Check first of all to see that the engine is well grounded by a "cable to the chassis, and that the chassis is well grounded to the battery! Either of these that is dirty, corroded, or terminal broken, - could cause a number of other problems too (but you might not have nonticed them), -- and it is not a definate "every time thing either! If this does not help, I would check behind instrument cluster, and make sure that the plugs are properly seated, and none of the connections are "dirty"!
By the way check your fuses, a dirty fuse terminal could make electric cut on and off too! ... When you lose a good ground, all kinds of funky thing happen, and gauges may really go crazy, - because of power feeding backwards form somewhere else to! Don't know how long you have had the problem either, but if you notice that it is diffreent in dry weather,- than it is when wet and humid, - this is also indication of loose or bad connections!