I have a 1992 nissan sentra andmy air seems to be working normally and gets good and cold as long as I am traveling 45 mph or less as soon as I go above 45mph it starts getting warmer and warmer this does not seem to be rpm related because it does not matter if I am in 3rd 4th or 5th as long as I am above 45 as son as I drop to 45 or lower it gets cold again low side pressure is at 45psi per the gauges any help please

thank youJohn B