420a Tranmission Problem

[MoarBoost]

So i got my 1999 gs about a week ago with 99k on it and it runs well except it doesnt wanna get out of 3rd gear. (automatic) it will go through 1st,2nd,3rd and no further unless i put it in OD. Its also very sluggish up hills, i checked the fluid its not burnt and looks normal. The car shifts fine, no hard shifts at all its really smooth actually. So im stumped. What could it be and what are my options

 

[Tha Fella Dylan]

I have an auto too. Do believe ours are only 4 gears my friend. The last gear will go if the o/d is engaged. Nothing wrong with it man.