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The car is a 2007 Mitsubishi Eclipse 2.4 l4, 50,000 miles equipped with an automatic transmission and sportronic.

At first I thought the issue was that the simple act of switching into third would lose 300rpm, but I discovered with some playing around that that's not quite the case. It's almost like it hits a pocket or bubble of some kind right around 3000 rpm while in third. If you switch up from 2nd at 4-4.5k you lose power for 2 seconds because it hits the bubble instantly before catching. If you switch below 3.5 and continue to accelerate you hit the bubble right around 2900, it loses the 300 and coasts for a second, then catches and pulls right through 3000 like it was trying to make up for it. If you bump up twice and go from 2nd straight to 4th (as quick as you can with an a/t) you skip the bubble but of course you have the whopping power of 4th at your disposal.

In a nutshell, no matter how you get there, there seems to be a pocket of empty space around 2900-3000 rpm in third gear. This seems to be affecting a lot of people with the same model year car as mine according to the Eclipse forums, but nobody has been able to get it fixed.

Any ideas? I could go leave it with warranty and be without a car for a couple of weeks, but I don't get the feeling from the Eclipse forums that they'll fix it.  

What is 4.5k? Do you mean 45 kilometers? If the trans was slipping you would be getting an increase in rpms not a loss. What you are describing sounds like either a bind up or an improper tcc clutch operation. If it was a bind up it would only happen during a shift {a component disengaging to slowly} so i think you have a glitch in the tcc application like maybe your getting full lock up for a couple seconds. You need to drive the car with a good scanner in data mode so you can monitor what the computer is doing when it happens.Particularly the tcc application.  

No, I mean 4.5k, or 4500rpm to be sure.

My scanner is pretty darn cheap, it reads codes and pretty much nothing else, but my neighbor has a fairly snazzy one that might do the trick. If so what kind of data should I be looking for when it occurs?

I'm reading up on the TCC and it looks like on some vehicles it's possible to simply disconnect it and run without doing any harm. Checking my service manual to see if I can do that, because if it's faulty I'm pretty sure I can get it replaced for gratis thanks to the powertrain warranty. I just hate going in to warranty not knowing where to point them because then they'll keep my car for a few days and return it having done nothing.  

I use a Snap-on scanner myself. The cheap scanner/code readers don't give you any decent trans data. You would want to watch all sensors and solenoids but mainly focus on the tcc duty cycle.  

Well if you're referring to the Solus... that is some beautiful equipment but way out of my price range. I drive a Mitsubishi, not an Aston. Turns out his scanner doesn't do real time data either, I was wrong, so I'm going to keep looking around and see if I can source one that does.

I appreciate the help, it's got me going in what feels like the right direction so that's a great start.