97 sonoma hard shift

just recently my truck (97' gmc sonoma 4x4 4.3 automatic) started shifting hard into second gear and third gear. the weird thing about it though is that it will shift hard for about a week then stop for a few days. Any ideas???

What is the mileage and service history?
Any check engine lights?
How is fllter and fluid and fluid level?
You need to get it on a transmission capable scanner and look for codes and read out all shift adptive and shift errow readings.
Post back any codes.
MT