Help, 89 Jeep cherokkee Auto shift problem

Help, I have an 89 Jeep cherokee 4.0L Asian Auto. It has started acting up... About half the time it doesn't shift out of 1st/2nd gear. It appears both on initial start and driving and also while on the road for a length of time. It has been sitting for about 2-3 months but driven every other week around the block... I have changed the fluid and checked electrical connections.... What could I haved missed / overlooked that might cause the problem? Any and all help / ideas are appreciated.

Thanks, Tim

well it your throtel position sensor located on the throtel body. i have an 89 cherokee same thing happend to me that fixed it right away. exspesive part 160 canadian.hope this helps