When I split audio on the timeline (not stitched, I'm inserting pauses), the audio glitches when it starts again. The only way I've found to fix it is to move the split audio onto another layer, which is annoying and messing. Is this a known issue?
I have had this problem also; there are a couple of ways to solve it (with differing amounts of work ;-). It is not an artifact of Camtasia, but of the way we humans perceive transitions in sound.
If you have the time, you can add "audio points" to the audio track that give you plenty of control over the volume. This allows you to fade in / fade out the volume as needed. Just position your cursor at the point you want to adjust the audio, right-click, and select "Add audio point" from the pop-up menu. (You'll want to have at least two points; one to lock the current volume, and one to modify.) You can then drag the point(s) up or down to vary the volume. The audio points are not really easy to see until you're zoomed in pretty far (see the screenshot below).
Alternately, you can try taking a one-second clip of ambient background noise from the recording and adding it to another track so it straddles the audio transition point. This works very well when cutting audio out and joining two segments of the recording. It is still "messing", but you get a cleaner sound.