The car has 230K on the transmission,...First check the fluid level on the transmission dip-stick, and make sure it is at the correct level, if the fluid level is low, add sufficient trans fluid to bring it up to operating level on the dip-stick, If the transmission still slips after you have filled it to the proper level, Then...You can try a QUICK fix such as changing the trans fluid and putting various types of additives into it,but sometimes that only makes the problem worse..Plan#1,..IF you plan on keeping the car as a DEPENDABLE DAILY DRIVER ,installing a RE-BUILT transmission or a LOW MILEAGE used transmission would be the sensible solution....,It could possibly be that, the planetary drive and clutch bands are worn out,.... when a transmission gets HOT and EXCEEDS the normal operating temperature,that's when they break down internally,DARK BROWN fluid that smells burned,and has tiny metal flakes mixed with the fluid are warning signs of a BURNED OUT TRANSMISSION..PLAN#2..If you plan on keeping the car as a second driver for going to work, or as an old beater to drive around,...you can try this at your own risk,...drain the old fluid from the transmission,(do not back flush the old fluid from inside the transmission housing),drain the old fluid from the torque converter as well,...re-place the trans filter with a new one,after the transmission pan and everything else is re-installed,fill the transmission with new fluid, make sure to leave it one quart low,warm up the car for ten minutes then add one pint of brake fluid to the transmission ,... then,Top off the fluid level the rest of the way with transmission fluid,let the engine idle in neutral for about twenty minutes,giving the brake fluid time to circulate inside the transmission, drive the car slowly for about two or three miles until the transmission stops slipping,.....or if the transmission completely burn's out....Then go to plan#1.