transmission trouble
 

hello out there, i have a 1990 Dakota 2 wheel drive that is having a problem shifting into overdrive this problem is intermittent time of day or temperature does not matter it may work fine for two or three days then it will get stuck in third for a couple of days, also i have noticed that when it will not shift into overdrive that the truck almost dies at stops but idle jumps back to about 600 rpm and does not die. when transmission is working fine truck does not try to die at stops idle is at about 800 rpm have check tps with ohm meter seems to be fine NEED HELP THANKS