Ive had tires that were almost completely unbalanced and you didnt feel a shake until 40ish... plus that wouldn't be intermittent at a constant speed. i have seen a broken belt (in tire) cause wobble even at 5 mph, but that usually feels like the apocalypse and again wouldn't be intermittent.

Only other suggestion i think makes sense was checking your suspension. Bushings, balljoints and so on.