It could be that you're not using the correct project setttings. Is there a red line on the timeline above your AVCHD clips? If so, you are not using the correct project settings and your project will need to be continually rendered.
If there are any red lines above your clips after you add effects or transitions, you should render those too (by pressing Enter).
You may also want to go to Edit/Preferences and set Timeline Scroll to Page or None.
Or it coulld be that your system is underpowered for working with AVCHD files (which usually requires a good, fast quad-core processor and full RAM load). But without knowing your system specs and operating system, it's hard to say.
One other aspect to check out would be the Thumbnail redraw. Try setting the Thumbnail Display (toggled in the Video Track's Header) to None, so that there are no Thumbnails for the Clips, just their names. PrE has about 3 - 4 variations of Display - None, All, First and First & Last (one of those might be PrPro only?). Many editors will only have the Thumbnail Display on in the very beginning of the editing, and will toggle over to None, when the Clips are placed onto the Timeline.
The ability to quickly generated the Thumbnail Display, and the other elements in the GUI, like the CTI (Current Time Indicator) will hinge on the video card, its driver and on the settings, like Hardware Acceleration, or OpenGL, etc..
As AVCHD is demanding material, to begin with, the overall power of the computer will often define things, like playback, as Steve has mentioned.
True, I only have a core duo and 2mb ram, 3 hard disks but everything else responds very quickly. I can
zip up and down a 1hr 25 min timeline with no waiting at all.
Rendering the area does seem to fix the problem but this gets destroyed every time I make a change to the area which is the reason I need the red line in the first place and uses up my time as well!
Looks like I better increase my memory.
I would think it needs much more resources to scroll the whole timeline than just move the red line.
When I change the timeline scroll setting it seems to be the movement of the timeline rather than the pointer that changes. I am quite happy to have the timeline stationary and the pointer scroll.
I only need the pointer when I am examining detail like noises or picking phrases of music.
The best result seems to be set to page scrolling.
When I set timeline scroll to smooth, the red line dissappears when the timeline is scrolling which is pretty useless.
Jumps in the pointer also cause brief gaps in the sound but the video playback is quite smooth as verified playing a slow zoom in.
What do you mean about incorrect project setting causing the red rendering line? I though this line would always show where you do anything that requires rendering?
I have set the project to the same as the video 1900x1080i 25 frames.
It would be good if there was a way of slowly rendering in the background while I am working on another area - like a background virus check?