Any suggestions, My Idle drops off and stalls when the AC compressor kicks in. runs normal when AC and heater turned off

Car runs fine and AC works fine when driving at nromal driving speed but when stopping it will stall
Could it be vacuum leak or a solenoid that has gone bad?? and if so where would I find that solenoid?

Thanks for any help someone could provide

If the 99 Diamante uses a similar idle speed control as earlier vehicles it will have an ISC stepper motor on the throttle body. with a 6 pin connector going back to the ECU.
Sounds like a problem in that area.

It should give a fast idle at start-up which then slowly settles back to normal, if that is OK then the ISC is probably OK and it could be related to a missing AC signal.

You might be able to hear the ISC run when ignition is switched on before starting the motor, ias it hunts for its initial position.