A/C problem in 99 Lumina

Started noticing cooling abated when car was stopped, such as at a stop light. Cooling returned with forward progress. Checked fan operation and find that the fans cycle on (low speed) when high side pressure reaches about 200psi. Once fans are running high side pressure decreases-not sure at what pressure they cut out, but they do. Advancing the throttle with the fans running (low speed) yields low side about 43 psi, high side about 225 psi. Never get high speed fan.

My suspicion is that the fans should be cycling to high speed almost immediately, but can't find any description of the conditions that command "high speed" fan, and so am somewhat unable to carry my troubleshooting further. On the other hand, perhaps my pressures are incorrect. Can someone lend a hand? Thanks.