Most often this happens when people try editing media that was not meant for editing, like something from a cell phone or downloaded off the Internet. The general solution is to stop trying to edit that stuff and only use footage you shot yourself using a pro level camera.
Does that apply here?
In most cases I'm editing AVCHD I shot with my Sony Handycam XR-520,
I've realized on Internet that Sony's AVCHD format is not as common as those shot by the other cameras,
but It played smoothly in PP CS4 (though sometimes lagged but overall it was acceptable.)
It is AVCHD, albeit using 4.0 profile @ 16 Mbps max, but it will be handled without problems by PR. It does require quite some muscle from the PC, but that is the case with all AVCHD cameras. Practical minimum is an i7-9xx with at least 6 GB, so that should not be the problem. Your problem may well be your disk setup. Raid0 for the OS disk is never advised, raid0 for the media and project files may be a solution, but better is to follow this: Adobe Forums: Generic Guideline for Disk Setup
In most cases I'm editing AVCHD
OK. Next question. Fully paid version, or trial?
I found my problem solved!
It's an issue of my Sound Blaster X-Fi card.
I used to set Creative ASIO as audio hardware,
but apparently it is not working in this enviroment.
I tried setting it back to Premiere WDDM,
all problems gone.
Thanks to everyone who replied!
They're good points to improve performance