I don't think increasing the RAM usage will solve this problem as it is for Dynamic RAM Preview. Not normal previewing (see the help file if you don't know what that is) . You don't give us your PC spec. so we don't know if it's even capable of running HD footage or even if your talking about HD footage.
Make sure your hard disk is less than 70% full and is not fragmented. What sort of disk is the footage on, internal or external USB, if the latter put the material on an internal drive. It's usually best to have the media on a separate HD to the OS and program files.
You can also reduce the preview quality to get your video to run smoother.