Automatic Transmission Shifting Problems

I have a 1990 Hyundai Sonata which has developed transmission problems, it will not shift smoothly between 1st,2nd and Drive (it bangs into gear) Also if it does make it into drive, it will drop into second with a bang after a very short period of time. When the car is cold it shifts smoothly thru the gears as normal, but as soon as the car warms up the shifting problems begin. Any suggestions? I have replaced the filter and the fluids with no change in operation.

I have a 1990 Hyundai Sonata which has developed transmission problems, it will not shift smoothly between 1st,2nd and Drive (it bangs into gear) Also if it does make it into drive, it will drop into second with a bang after a very short period of time. When the car is cold it shifts smoothly thru the gears as normal, but as soon as the car warms up the shifting problems begin. Any suggestions? I have replaced the filter and the fluids with no change in operation.
My automatic does something similar. When warmed up the tranny will randomly drop into 3rd and stay there. Haven't figured it out.

ITS THE INPUT SHAFT SENSOR. or the ouput shafter sensor or both. Well at least we're not in the dark anymore. I went onto webtech and searched for "speed sensor" under 1999 sonata 2.5l and came up with the following: DTC - P0720 (OUTPUT SHAFT SPEED SENSOR SYSTEM)
Code No. P0720 Output shaft speed sensor system
If the output from the output shaft speed sensor is continuously 50% lower than the vehicle speed for 1 second or more while driving in 3rd or 4th gear at a speed of 30 km/h or more, it is judged to be an open circuit or short-circuit in the output shaft speed sensor and diagnostic trouble code No. P0720 is output. If diagnostic trouble code No. P0720 is output four times, the transmission is locked into 3rd gear or 2rid gear as a fail-safe measure.
Probable cause
Malfunction of the output shaft speed sensor
Malfunction of the transfer drive gear or driven gear
Malfunction of connector
Malfunction of the TCM

DTC - P0715 (INPUT SHAFT SPEED SENSOR SYSTEM)
Code No. P0715 input shaft speed sensor system
If no output pulse is detected from the input shaft speed sensor for 1 second or more while driving in 3rd or 4th gear at a speed of 30 km/h (19 mph) or more, it is judged to be an open circuit or short-circuit in the input shaft speed sensor and diagnostic trouble code No. P0715 is output. If diagnostic trouble code P0715 is output four times, the transaxle is locked into 3rd gear or 2nd gear as a fail-safe measure.
Probable cause
Malfunction of the input shaft speed sensor
Malfunction of the underdrive clutch retainer
Malfunction of connector
Malfunction of the TCM

I have also had this happen to me like two times:
Automatic Transaxle Diagnostic Trouble Codes P0715 & P0720
DESCRIPTION:

An improperly functioning input speed sensor or output speed sensor on a 1999-2001 Sonata, 2001 Elantra, 2001 Santa Fe and 2001 XG300 may result in the following conditions:

Harsh P-R or P-D engagement shock
Transaxle allows acceleration in 1st gear to 5000 rpm, then shifts into 3rd gear fail-safe
Check Engine light on

So now I just have to recheck these pages again to see if I will be able to check these sensors myself. I plan to replace both if they are inexpensive. Check my posts at under drivetrain at sonataspeed.com for further info.