A/C Code 66 (low freon) at speed, but not a idle.

Getting a code 66 (low freon) on my 1992 Regency Elite after conversion from R12 to R134a. Pressure is good, so I replaced the a/c pressure switch on the accumulator suspecting it to be the culprit. Car can site at idle for 30+ minutes, 60 degrees max fan, without an issue and compressor cycling at 10 secs on, 2 off. But as soon as you drive away within a mile it will gen a code 66 again. Been through AllData without any progress. Any comment would be appreciated.