Okay, so recently this problem started, this is within 2 weeks of a tranny flush. I get hard shifts with varying degrees of of harshness and an occasional growling/grinding sound in park, neutral, or when coasting. I also got a very weak and rough acceleration only once at WOT, i have no idea whatit could be. I checked the fluid level it was fine and the fluid smelled fine. I asked aroudn the shop, one guy suggested maybe the valves, does anyone else have any other ideas? Oh, and the shift into OD is fine.

Transmission problems are seldom something a home mechanic can deal with.
I used to work for AAMCO and know from experience that trans problems usually require that the thing comes out of the car and be torn down to find problems.

Even messin with a valve body is not wise while it is in the car cause you pull the valve body in GM FWD and all kinds of neat little check balls come falling out.

Being a 96 SSEi, you have the 4T60E. That trans is still vacuum-modulated, and the modulator can affect shift firmness. Especially on a cold morning. You might want to consider installing a new modulator (even an aftermarket adjustable one).

Expect to 'feel' shifts alot more in the cold weather. On all transmissions, but particularly on those with high miles.

I don't know about his particular one, but is it possible it has a vacuum line going to this modulator? If so, check it out first.

There is a vacuum line going to the modulator. Like Bill is saying, you may also want to check the vacuum lines.