Your compresser is running faster while zipping down the road and brings the pressure up. Get the system tested (or buy the gauges) and fill it up properly. Too much will blow out the safety plug. If you spot any oil stains on the A/C lines that is where the R134a is going. I've seen quite a few Goodyear hoses leak. The Stop leak/compresser oil combo sold in stores works well on the hoses. Added some to wife's car over 2 years ago and mine last spring and they still cool well.
