Possible clutch replacement needed

It started in mid-December, when I would shift into fifth gear, accelerating harder to pick up speed and merge the rpm's would go about 1,000 higher than needed, then settle back to where it grabs properly. Then I started noticing it happening in fourth gear too. The only other problem I have seen was an engine code for the coolant temperature sensor, but the light went off without changing it. Would it be the full clutch or the slave cylinder that needs replaced? Or something else entirely?