Do I need a new transmission?

When I Press on the gas the rpms go all the way up
to 5 and the car barely moves, it sometimes jerks
when I'm pressing the gas. Once I get up to 40
miles per hour the car drives perfect. Somedays it
works perfect somedays it acts up... is this my
transmission? I heard once a trans is done it's
done, there's no good or bad days! Can someone
maybe help me out? I don't want to spend all this
money for a new transmission if I don't need it!