My initial thoughts are that it's in your head, if I'm honest.
Easy way to check though, do some timed runs between 30 and 70 in 3rd gear and see if there's any noticeable change. If there is, then you know you have a problem somewhere. No lights or clutch slippage sounds to me like a mental thing though, and the current temps (in the south, at least) won't help with power.