Welcome to the Board!
As for your question, it is very difficult to give any sort of definitive answer since you provide so little information. Please read the sticky post up towards the top of this Board - the one labelled "Please Read This Before Posting", and come back with the sort of information it requests, and I am sure someone should be able to help you more.
As a general comment, though, the preview screen has never been one of the strong points of Video Studio. In Clip mode, you are merely playing back your original video as it was filmed. In Project mode, you are playing it back with all the edits you have done to it. And to do this, Video Studio in effect has to process the video, cuts, transitions, titles, background music etc, all together and in real time. Depending on how much editing you have done, and factors such as the format and size of your original video, the resulting project preview will not necessarily be high quality. If you want better quality, then in File > Preferences > General, you need to change the Playback Method to High Quality Playback, instead of the default Instant Playback. Be warned, though, that this will take some time as the program then does a full, though temporary, render of your project.