What would cause 350 turbo tranny to shift from first to second at 35 - 40 mph and from second to third at 55 but still down shift fine?

if you just started driving this vehicle(recent purchase...etc. ) a high speed torque converter may have been installed for fast accelleration or a different valve body in the

transmission could have been installed. if this recently started, start with new fluid and filter in transmission. not difficult to do on GM rear wheel drive. buy a repair manual first and go at it, or expect to pay 150 up.