The whole speeding up with no change in RPM thing should only happen in 4th gear. When I say "4th", I mean when the tranny is in 4th... Not just when the gear selector is in "D". Typically, top gear and torque converter lockup mean that you're going over 50mph.

It's fine the way it is, so long as it only does it with very light throttle. Moderatel to liberal throttle application should cause the TC to unlock, or the tranny to downshift. It's supposed to do that.