This is only speculation. Any diagnosis without readings from an A/C gauge set is speculation.
First, Theory of Operation: The switch you replaced on Accumulator is a Low Pressure Cutoff (LPCO) switch. It releases your compressor clutch when Low Side (Suction) pressure falls below about 20-PSI. It does that for at least two reasons:
1. Pressure needs to be above around 20-PSI because the evaporator core temperature will fall below freezing.
2. Below around 20-PSI means the refrigerant charge is low. It may freeze the evaporator core, starting at a corner and working across, but only part of the core is cooling. Also, the compressor isn't being adequately cooled by returning refrigerant, and the lubricating oil may also not be circulating.
Speculation: Your system is just a tad below minimum charge level. At idle, compressor RPM and engine fan air flow aren't enough to pull the low side below 20-PSI. On the road, the compressor runs fast enough, and the condenser is cooled well enough, to pull below 20-PSI and the LPCO stops the compressor.
Please note: 20-PSI may not be the exact value. Looking at a low side A/C gauge, you can follow the R134A curve, and see what pressure causes temperature below 32*F. Low side pressure has to stay a few points higher than that.
