Try holding the brake pedal up with your toes. If the torque converter is unlocking and relocking, a misadjusted brake light switch can cause that, and holding the pedal up will confirm that if the problem stops. You can also duplicate that to verify that's what's happening by holding the gas pedal steady at highway speed, and momentarily tapping the brake pedal with your left foot. Each time you do that the torque converter will unlock for a few seconds and you'll see the engine speed go up a few hundred rpm, then drop back down.