If I'm reading this right, you're stating that the car only runs bad below 3000 rpm? Does it ever get above 3000?
Has the car ever run properly since you've owned it? If so, what was the last thing done to it before it started running this way?
Take your time, be clear and don't try to diagnose anything, just relate the facts.
So far it appears that either you have a t-belt problem (installed incorrectly or something is wrong that caused it to jump a tooth) ot possibly one or more clogged convertors. Maybe a fuel pump is going bad and not supplying enough fuel.
The SelectMonitor only deals with electronic components and not mechanical issues, that's why it's not 'seeing' anything wrong.
Do not go back to that dealer, it's readily apparently they don't know their ass from a hole in the ground. Nobody tells someone to sell their car because they can't figure out how to repair it. They might recommend selling it because repairs will be more than the car is worth, but that's another story. They haven't even figured out what is wrong, so that's not an issue.
Even if they changed the t-belt I'll bet a dollar to a donut that they're too lazy to pull the covers and actually look to see if the belt is installed correctly. That would take at least 15 minutes and they just don't have that kind of time... I've worked with guys like that and I know their modus operandi.
Don't worry about the gas the previous owner used, while it may have caused a performance hit for her, it's seriously doubtful that it would have caused any long term problems. If it had you never would have bought the car in the first place.