RPMS drop while crusiing
 

Have a 68 442 that has a highly modified engine setup. While cruising at about 3500 RPMs on the freeway, RPMS dropped to under 2000 for no apparent reason. When I hit the gas pedal they came back up only to drop again intermittently.
This happened after a long (200+ mile) aggressive drive where I opened her up more than several times. Any idea what this could be from?