Yah, if the hi-pressure switch is cutting out then either
A: You're not getting the cooling air you need
B: there's a blockage somewhere. (Not probable or it wouldn't work in town)
Air flow is hard to understand, what seems logical is sometimes waaayyyy off.... Heck, you could be getting reverse (back to front) air flow patterns over that coil at highway speeds for all you know!! Takes alot of trial and error (or engineering if you have the skills) to get a setup like that going correctly.
