Sounds like a transmission problem, then. Take it to a professional with diagnostic equipment that can tell what's slipping internally and they can advise you much better. Not much we can do on a forum except list all the things that might be wrong with a transmission, and that's not very helpful if you're going to a mechanic anyway sooner or later.
Our questions were very specific, and everything has some meaning. For instance, Delco 41-103 are the ONLY plugs that have been shown to work well in the I6 engine, according to hundreds of posts over 8 years on three forums that deal with this vehicle. Since a normal "tune-up" doesn't apply to this engine (no plug wires, for instance), can you confirm that you now have 41-103 plugs?
If you ever had a P0128 code, and didn't change the thermostat right away thinking it was no problem to continue running cold for a while, you might have started to clog the catalytic converter, but that would cause symptoms at high RPM more than low.
But a slipping transmission is pretty specific. Could be a simple solenoid, filter, pressure problem, or more serious internal wear/damage.