Automatic Trans Slipping

Well I'm far from an expert when it comes to automatic transmissions, so please bear with me.

Anyways, the transmission has always been maintained to schedule. Today on the highway, I experienced something I never have before. I was going about 60-70 mph in 4th like normal, when it started downshifing itself into third, then sometimes neutral, and even as low as second. It will downshift itself to whatever it wants, and sometimes hang there, other times shift back up. Sometimes it does this back to back, other times it's completely random. One time it downshifted into second by itself it was at about 5,000 RPM, I actually gave it WOT to see what would happen. It revved up to it's normal shift point close to redline and shifted into third normally, after which it hung into third for a long long time before shifting back into 4th. I still have proper use of the gears, 1-4 and upshifts seem normal (no clunking or noises or jerks, it's actually quite smooth for its age). Reverse is still normal, and engegement between P/N into any drive gears is also smooth and quick.

I plugged in my scanner, and there is no codes at all. Not too long ago I discovered this code, P0705
(http://www.elantraxd.com/forums/showthread.php?44443-P0705) that kept coming back over and over (despite not throwing an actual CEL). However, without me doing anything to it, the code disappeared and never returned.

I tried resetting the ECU/TCU by removing the negative terminal for 30 min and that didn't change anything. I also checked the ATF with it fully warmed up, in park and the fluid is at full, still looks and smells good.

Any ideas? I can't really drive the car anywhere, as my commute requires higher speeds and I'm not thrilled with it downshifting into second and driving with the RPM's through the roof.

Any help is appreciated.