[85MCSSFREAK]

i got a 85 monte carlo ss with a 305 in it with a thm200r4. everytime i go up a hill the car will be running at 1800 or 1900 rpms in like 2nd or 3rd gear and then all of a sudden it drops rpms down to like 900 to a 1000rpms i think its slipping into the next higher gear at too low of an rpm. what do yall think and is there anyway to fix it?