Stumble? at 60+

Was driving my Valiant on the freeway yesterday when I noticed that the tach would drop a couple hundred rpm kinda suddenly and then recover. I noticed a bit of a vibration in the car when it was happening, but at 65 there's a lot of vibrations in that car, so I'm not convinced I wasn't making it up. Pulled off the freeway and finished my trip on the highway, kept it under 60 the whole time and never had that problem.
It didn't sound to me like the engine was missing, and didn't feel like she was down on power when the tach was dropping. I think maybe the ground on the tach was vibrating loose, but unsure, I'm gonna go check it out today, pull a couple spart plugs and double check the grounds to the ign box and the tach.
Running FBO ign box, and a cheapo sunpro tach.
Any input on what this could be would be great.