you have a leak some where. when your driving down the road the low side pressure drops as the wind cools down the condenser, also the higher rpms the faster the compressor is going so less pressure thru the lines. then as you slow down the pressure starts to rise again then hits thats point where it turns on the compressor, boom cold air. (of course with the compressor engaged) but on the gauge on the low side its supposed to be around 35-40 with the size truck you have and high side around 150-250psi on a good charging system. but yea take it to a shop and they'll look it over