AC Freeze Up

I have a 2005 Endeavor with 130k miles. It's been a great vehicle for me and I intend to pass it down to my high school son. The problem I am experiencing is that, when travelling for more than 2 hours at highway speed, the a/c stops blowing any air at all. If I turn the a/c off and keep the fan running, air will begin blowing again along with small ice crystals. I am assuming that the unit is freezing and then thawing. I can then turn it back on for a while until the process begins all over again. I've had the system drained and recharged and still experience the same problem. I'm not a cooling guru, so any help you can give would be much appreciated.