Sometimes Jerks Into Second

[Newlin]

Driving around I've noticed that on some occasions my car will rev further than it usually does in first (up to 2.5k or more, when it usually shifts around 2.2k) and then quickly jerks its way into second. It shifts the later gears just fine. It doesn't do this often and when I feel it is about to (revving too high) I just momentarily let of the gas and when I reapply pressure it shift nice and smooth into second.

I haven't found any rhyme or reason as to why it only does this some of the time. I am however taking it into the shop next weekend to get my transmission fluid replaced and an axle replaced so I can hopefully get this taken care of then as well.

Oh, it's a four speed auto trans.