'95 buick riviera s/c won't shift into overdrive

[johnd89]

Hi,
My car shifts correctly 1 to 4, but after some point, on the highway it stays into 3rd. Is this a common problem for those cars or 4T60-E tranny?
What should I check first?
PS: sometimes, the 3-4 shift is bumpy. The car is pretty high miles (130k), and I changed recently the filter/fluid, but this didn't change this behaviour any.

Thx for any help!