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?

-RC