The least I would do is, drain and replace trans fluid. Make sure the fluid level is correct. Recheck level a few times to be sure. Sometimes it's kinda tricky.
This would be a great starting point.
I would not do a power flush at this point. I think the milage is too high for a power flush.
Try new fluid and post back results.

J-Bird

'95.5 T, SR5, Bone stock w/32x11.50's
'06 Sonata LX