what causes my gear transmission to go off on gear 3 or 4 on speed ? on 2002 Lexus ES300

once this happens I reduce speed and it returns to the low gear when i increase speed it goes off again am really troubled about this, i took it to run a scan with an error code reporting " gear speed sensor not found" could this be the problem ??

Asked by for the 2002 Lexus ES300
take it to a tranny shop for diag