I have a '00 Pontiac Bonneville which has an intermittent vibration at speeds above 65mph. I’ve had the brakes inspected, all the wheels balanced, and had an alignment performed. Although the car now rides better overall, the root problem still exists.
The vibration comes and goes in 5 or 6 second intervals and is felt moreso through the seat/inside panels rather than the steering wheel. I had the had the brakes inspected since it sometimes felt more pronounced during braking but the mechanic said the brakes checked out fine.
While I am aware you said you had the wheels balanced, there is STILL a possibility the tire can go out of balance over the 65 MPH range.
Up to what speed were these wheels and tires balanced?
There MAY be a tire starting to become separated. You say you feel it moreso in the seat than the steering wheel so I’d start with the rear wheels first.
Now, because tire separation is difficult to see (especially if it has just begun) you may have to use a spare (of the same size and tread) and relocate it around until you find the culprit.
This MAY not be the problem though as other things like worn wheel bearings/worn transaxles or worn engine/trans mounts will cause vibrations too.
Along this line of thinking, find a shop that can do " ROAD FORCE BALANCING " and it whill help see a high speed tire issue that spin balancing will not.