you'd hear EMI in your headphones or speakers long before it screwed up the electronics in your md player. it's almost certainly vibration. it could be that around 70mph, physical vibration hits the perfect frequency to really screw things up.another possibility is something is screwy with your car's electrical system (alternator, voltage regulators) is causing spikes/dips in the power going to the player. Yet another possibility is a ground loop, caused when one of the paths to ground has a significantly different resistance than another path.
I had a ground loop once in a speaker connector. If you turned the volume up too high the whole system would squeal. I lost a lot of hearing to that one
is this device mounted in your dash, or is a portable? if it's portable, try holding it in your hand, in your lap, or putting it on a sweater to minimize vibration.
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