Sounds like your throttle linkage was not adjusted correctly so when the second gear band came on, the line pressure was too low and you kept slipping the band. They do require occasional adjustment but not that often. When you switched to a manual shift valve body it fixed your problem with a line pressure increase and eliminated your throttle valve issue.
To get a clean 1-2 shift with a band on valve body you also need a better solid reverse servo such as the A&A that also increases the release spring pressure to get the band off quickly.