Transmission problems

I have an -88 Astro with a TH700 that from the start didn't work in any gear. I ripped it apart and changed the band and all disks. After this rebuild, the transmission worked for a week. During this week it shifted a bit hard, but as time went the shifting got smoother. The problem is that it after this first week got so smooth that it never shifted from second to third gear... The slipping seemed to get worse when the ATF fluid got warmer. I also noted that sometimes the transmission seemed to start on second gear instead of first (when I floored it, it kicked down to first...). Could these two problems be related? Except these problems the transmission works on reverse, first and second gear.

Any help would be greatly appriciated!