Looking at vehicles to buy and came across a 2004 rx-8 manual transmission that looks like it is in good condition. His post says that it sometimes won't go above 50 mph. I will ask him more about it if/when he replies. My assumption would be a bad clutch, but was wondering if anyone has other ideas. Also could anybody who has replaced a clutch on this car before tell me how much it cost for them or if they did it themselves how difficult it was on this car?

 

It could be a number of things, there's not enough info here. Could be the cat. Ask him to describe what actually happens when he gets to 50.
Clutch wouldn't be high on my list of candidates, but its easy to test when you test-drive.
Read the rx8help.com section for New and Potential buyers.

 

As stated above, it could be many conditions that could cause this. To add to the above, the car could also be in limp mode. A common cause is a bad oil metering pump or bad connector at the metering pump. I would advise calling to obtain more info.