When I drive my Mustang, When I'm between 25 and 45 (approximately) MPH, the transmission feels almost like it's sticking ever so slightly. Where sometimes I have to rev the engine a bit if the speed limit is 45mph to get it to go into the next gear. Any ideas?

 

Bring it to a shop, too vauge of a description to diagnose over the web. A lot of things could cause the problem you're experiancing, however most likely culpriate is a bad sensor, but then again, it could be a few other things.

 

Will that show up as an error code on the engine computer?

 

Not always, bad sensors is a very vauge term. Just because a sensor is bad doesn't mean that it will trip the check engine light.