Scotty. I have a 93 Nissan 300zx n/a automatic.

Last year I had the transmission rebuild. Because it was locked in neutral. This month I started using the car again. But I have a issue. The car is not shifting into 2nd gear when it should.  The rpm will go up to 4 or 5k and it won't shift into 2nd gear or will lag.  But if I go by 3k rpm and let go of the gas pedal . And then give it gas again it will shift into 2nd gear with no problem.. I don't know what it could be. Any ideas. Also when I floor the gas pedal it will hit high rpm but it lagging  to respond and make shifts.  What should I try to check.. could it be a bad torque converter? Engine timing ? What could cause a lag in WOT. Transmission. Please any idea will be great.