Transmission Troubles

[nessies2003]

My 89 Toyota Supra Turbo (automatic) has been showing some signs of transmission problems, but I'm pretty sure it was replaced in the last 30k miles.

About 6 months ago or so, the engine started doing this sputtering thing right after ignition until the car had run for a bit. That issue hasn't really been a problem and could not be duplicated at the mechanic.

Recently though, when I'm driving about 35 mph, the transmission tries to switch gears and never finishes. The engine just keeps reving and accumulating rpm with no gear switch. I've found that turning the Overdrive off has allowed me to drive when this happens, but it is clearly not a permanent solution.

I checked my fluid today after driving home on a reasonably flat surface, and it looks like it is slightly low for being hot (at the bottom of the HOT marker). It's also brownish.

Is it possible that I just need to add or replace fluid, and if so, should I get a full flush? Any other possibilities?

Thanks.