2004 Chevy Trailblazer Shifting gears, also RPM gauge

Question: I have a 2004 Chevy Trailblazer LT, with about 67k miles on it. My Trailblazer (automatic transmission), when between 4th & 5th gear, does not seem to be able to differentiate which gear it should be in - it will go back & forth in a somewhat jerky ride as its shifting up, then down, and not able to go smoothly. When I'm on an incline (even if only slight), it's at its worst. But happens when on a flat road at times, too. If I step on the gas and force it to accelerate in the lower gear it will finally catch, but that is not normal! I took it previously to a dealer and when hooked up to the computer it did not show any problems - but there obviously is a problem. What should I do?

Also - I'm not sure if it's related, but my RPM gauge does not always work properly. Sometimes it functions normally, but sometimes I will be cruising at 60 mph and it is down at 0 rpm. Or at times it has even been off the gauge, past 7 RMPs.