my dad's 93 ls400 has this problem only when the AC is one. he drives up to park his car and once he shifts into "P", the car stutters a bit with the rpm dropping between 200-400 rpm for like 2 seconds, and then goes back up and the car is fine. anyone with this problem?

 

The A.C. draws/needs a great deal of power to run so it most likely is a bad battary but could also be his alternator. Have the battary put on whats called a "load test" first to see if it's holding a charge before you change anything. Any mechanic could do it, takes a few minutes and should come back to a reading of 12 volts or so. If it shows a good charge then its the alternator. Also Autozone will do it for free, just bring the car to them and ask them to do a load test.