AC questions on 420sel

[pexcellence]

I am experiencing an intermittent problem on my A/C. Charged it up last year. Low side 40 mm Hg, high side 220 mm Hg at idle. The A/C seems to cut out at higher speed and would mysteriously come back some of the time once speed is slower. Sometime would stay on at freeway speed. I am thinking that it could be the speed sensor. Is it also possible to develop high pressure at increased speed due to an overcharge condition, therefore causing the pressure sensor to cut out. I think the cut out pressures are 40-50 psi for low and 150 psi for high. I can put a manifold guage on and take pressure reading while flooring the gas.

What is the conversion from psi to mm Hg?

Thanks,

John

P.S. A/c blows cold once it is on.