Well, the good news is that, from Adobe at least, the trial versions are identical to the ones you buy, except for the time limitation.
That sort of clicking/stuttering is generally one of two causes: either your computer processor running out of capacity or your hard drive not being capable of enough throughput--and the processor side is the most likely issue.
A few things to do:
Shut down everything unnecessary running in background, with special emphasis on networking, wifi and anti virus software. If you get really keen, do a Google search on "optimising computer for audio" for your operating system. There are lots of advice sites and you'll get some good ideas. A programme like "EnditAll" can help with this.
Download and install the latest drivers for whatever sound interface you're using.
If you have a choice, use ASIO drivers rather than MME. If your interface is MME only, download and install ASIO4ALL (free) and select that.
In the Audition Edit/Preferences/Audio Hardware screen, play with the latency setting. Higher latency equals more reliable playback (but more delay when recording--which is why direct hardware monitoring on an interface is a good thing. If you can set to ASIO under Device Class, you may have to adjust the latency in the control panel for your interface.
In the longer term, for audio use, it helps a lot to use 2 different hard disk drives--one with the operating system and your DAW software, and the other for the track data. An external drive (USB or Firewire) works well for the tracks but make sure the USB bus isn't being shared with other things.
FYI, my laptop is 4 years old now and I can regularly playback 24-32 tracks. It falls apart quickly if I load up too many real time effects but there are ways around this too.
Hope this helps.
That's what I do. I drink and I know things.
-Tyrion Lannister (and Bobbsy)