I use to have the same trouble. I've posted to this question quite a few times. If you do a search you'll find this a lot and what people have done.
You have to have the shift shaft clawed centered on the shift star like stated above. Only thing is, it may look right but the cam (looks like a flat head screw) may be loose. Mine was. Whenever I shifted under hard exceleration it stayed in the same gear, but it shifted fine if I was just cruising around. That may be the your trouble. Tighten it and use the lock washer that's under it.
Also, my cam screw had groove in it. I grinded the sides of it flat. It still is effect for adjust but a little more touchy. Once you tighten the lock nut, it doesn't matter anymore.
Good luck.