94 Ford Bronco- Randomly shifts very hard. Replaced transmission to no avail

I have a 1994 Ford Bronco (351 V8 Eddie Bauer Edition), and the transmission intermittently shifts very hard (shifts at 3500-4000 rpm instead of 2500-3000 rpm). The problem seems to happen more frequently when the weather is warm, and the hard shifting stops after a day or so. About 18 months ago when the problem first appeared, I went to a few auto repair shops and they all said the cause was a worn valve body in the transmission. I then went to a Ford dealership, and they also recommended a new (rebuilt) transmission. I had the transmission replaced, and within a month, the hard shifting occured again. The dealership thought the transmission just needed some special transmission additive, but within a month, the hard shifting happened again. They were never able to pull any codes from the computer, and decided to replace the ABS sensor located on the differential. The transmission worked properly for a year, but started to hard shift again this summer. They replaced the ABS sensor again, but the problem returned a week later. Any suggestions would be much appreciated.