My 1997 mitsubishi galant idles at 1000rpm. soon as on gear(like drive/reverse) rpm drops to 500 rpm and runs rough. if I run the A/C the engine runs rough and seems like it has no power to move on drive or any gear

SOURCE: tune-up; low RPM at idle

you may have a bad plug or plug wire its hard to say for sure but id look at all the wires and make sure none of them are grounding out anywhere

SOURCE: 2001 Mitsubishi Galant Transmission problem?

Could be a few things:

1. Low on auto transmission fluid; this would cause you to lose a few of the forward gears (reverse is used less often so its the last gear to go Start the car and add fluid to the specified markings on the Auto Transmission dipstick
2. Bad auto transmission fluid; Check the fluid, if its brown and gives off a burning smell, it needs changing along with the filter
3. Bad transmisssion; if neither of the above are true, then it sounds like the clutches have burned out on the transmission or the transmission itself is bad; if 1 or 2 hasn't solved your issue the car needs to be diagnosed at a reputable transmission shop; Make sure they lay out all of your options and if you decide to have a major transmission repair done, the Kelly Blue Book value on a similar car similarlly equipted in exellent condition is about $6,800 retail

SOURCE: I have a 1992 Mitsubishi Mighty Max v6 3.0. It

Yes, the computer does retain fault codes. They can be retrived either with a scan tool or by using an analog volt meter. (Good luck finding one of those if you don't already own one.)
Volt meter method:
Find the diagnostic connector located next to the fuse lock under the left side of the dash
Connect the positive lead of the volt meter to pin 1 on the diagnostic connector.
Connect the negative lead to pin 12. Turn the ignition switch on and disclosure of the ECM memory will begin.
The ECM will output 12 volt pulses. There are long pulses and short pulses. The long pulses count as tens and the short pulses count as ones. Example: 3 long pulses and 2 short pulses means code 32.
If there are 2 or more fault codes in memory, they will be output one at a time in order of the code number from lowest to highest.

Code Description
_____________________________________________________
11 Oxygen Sensor or Circuit
12 Airflow Sensor or Circuit
13 Intake air Temperature Sensor or Circuit
14 Throttle Position Sensor or Circuit
15 Motor Position Sensor or Circuit
21 Coolant Temperature Sensor or Circuit
22 Crank Angle Sensor or Circuit
23 Top Dead Center Sensor or Circuit
24 Vehicle Speed Sensor or Circuit
25 Barometric Pressure Sensor or Circuit
41 Injector or Circuit
42 Fuel Pump Control Circuit
43 EGR Temperature Sensor or Circuit