Speedo question/problem

[glock35ipsc]

I have been driving my 70 4x4 this week. Been doing alot of work to her to get her ready for my wife to drive to work when the weather turns bad.

I noticed that the spedo is off exactly 5mph compared to my GPS. Nothing surprising there. But I have noticed that every once in awhile, it will just drop about 5mph, then go back up. The needle doesn't really bounce, just out of the blue it will drop for 5 seconds to a minute, then pop back up.

I am sure the 5mph off thing is in the calibration, but is the up and down thing a bad speedo? The odometer quit working years and years ago, so I wouldn't feel bad about throwing another one in there.

Thanks for any suggestions. Also, anyone know the weight of a complete disc brake front crossmember?