93 Del Sol AC Compressor Cycling Excessively

I have a 93 Del Sol S (no power steering) that the AC has started acting up at idle. Sometimes the compressor clutch cycles on and off
every few seconds when idling. It happens when the engine has been reving and drops quickly to idle, such as pulling up to a stop sign or light. The idle will run up and down between 500 and 1000rpm as theclutch engages/disengages. Eventually the clutch will remain engaged and the idle will stabilize at 700rpm. The system works fine at higher revs. The clutch cycling appears to be due to low pressure.
I've had the car for about a year, AC has worked great. The sight glass is clear except a few bubbles when the AC is first turned on. I hooked up gauges and pressures are normal according to the FSM (1500rpm/ 275psi hi / 30psi low/ 95F outside / 54F vent ). Most of the time at idle the clutch doesn't cycle (700rpm /220 hi /41 low / 60F vent). There appears to be an ample charge of R12 in the system.
When the clutch starts kicking in and out the lowside pressure will drop quickly down to <25psi clutch disengages then pressure goes up to 40 psi clutch engages, pressure drops to 25psi repeat repeat. This doesn't happen all the time, typically when revving the motor and allowing revs to quiclkly drop to idle. If the AC is turned on at idle this doesn't happen, likewise if the revs are eased down to idle.
Car has 142k miles on the original AC. I assume it's been topped off sometime during it's life. I'm reluctant to add more R12 to the system when pressures are normal at 1500rpm and the sight glass is clear.
Am I misreading this and it's just low on R12, or is it something wonkier like a bad expansion valve?
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