I suspect you may be using outdated hardware. Usually this kind of problem exists if you have a slow computer.
Here's what I think is the minimum PC specs required for digital video.
1) Windows XP: Some new PCs might come with Windows Me (Millennium Edition). Windows Me has some fundamental problems with stability and memory management that (in my opinion) make it unsuitable for digital video work. Windows XP, on the other hand, is very efficient and stable. Upgrading a Windows Me machine to XP is often challenging, so I recommend that you buy a PC that already has XP installed.
2) 512MB RAM: Video editing requires a lot of random-access memory (RAM) â€” the more the better. As if you didnâ€™t already have enough acronyms to remember, some PCs have a type of memory called DDR (Double Data Rate) RAM. DDR works twice as efficiently as regular RAM, so a computer with 256MB of DDR RAM will work about as well as a computer with 512MB of other types of RAM.
3) 32MB video RAM: The video image on your monitor is generated by a component in your computer called the video card or display adapter. The video card has its own memory â€” I recommend at least 32MB. Some video cards share system RAM (the computerâ€™s spec sheet might say something like â€œintegratedâ€ or â€œsharedâ€ in reference to video RAM). This tends to slow down the performance of your computer, so I recommend that you avoid shared video RAM.
4) 1GHz (gigahertz) processor: I recommend a processor speed of at least 1 GHz (equal to 1000MHz) or faster. This shouldnâ€™t be a problem because there arenâ€™t too many PCs still being sold with processors slower than 1 GHz. It really doesnâ€™t matter if the processor is an Intel Pentium, an AMD Athlon, or even an AMD Duron. The faster the better, naturally.
5) FireWire: Not all PCs come with FireWire (also called IEEE-1394) ports. You can upgrade most PCs with a FireWire card, but buying a computer that already has FireWire is a lot easier.
6) 60GB hard drive: When it comes to hard drives, bigger is better. If you plan to do a lot of video-editing work, 60GB is an absolute minimum. Sure, it sounds like a lot, but youâ€™ll use it up in a hurry as you work with digital video.