are you saving the "previews" to a network drive or the cloud? If so, I think that may be the problem. If that's not the issue, Please tell us your system specs: OS version, Premiere version, amount of RAM, Hardware specs including graphics card and your source properties and sequence settings. Another thought is that there might be something dicey going on with the hard drive the previews are stored on. Try changing your previews location to another drive if possible and at the root level of that drive and see if that changes the behavior.