I don't know if the RX7 transmission are electronic or not but I'll base my opnions of my thunderbird's transmission.
On a 4R70W Transmission, if it shifts too early, a few things could cause it;
1) Not Enough/Too Much fluid
2) Bad MLPS (Neutral Saftey Switch)
3) PCM (Power Control Module) - basically controls everything in the tranny.
I wish I could help you out more!