I have a 400E with the exact same issue. Driving from the Tampa area to Atlanta it freezes up after 4-7 hours or so..every time I make the trip although this only started last year. Never before. When I got out of the car to look under the hood to see if the compressor was engaging, I saw the low pressure side in a case of ice. Maybe the evaporator sensor is at fault although my system also cycles under normal circumstances. I wonder if the cycling I'm getting under normal driving is because of the high pressure switch cycling it as opposed to the evap sensor causing it to cycle. I'll be interested in your results.
Brian