Re: serve help
Your wrist got tired, and your wrist snap was later, causing your serves to go long. If you're going to serve big, you need to work on your fitness, or else your serves won't be usefull later on in a match. Try wrist curling when you're not playing, and when you are playing, make sure you snap your wrist at the right point.
Also, it's possible your ball toss was errant, but that would give you mixed results, rather than consistant long serving.