Sounds like you have Adaptive Resolution turned on. There's a little menu below your Program monitor on the right hand side. It has several options: Full, 1/2, 1/4, etc. I'd bet that you have it set to 1/2 or 1/4. Premiere uses this setting to save processor power by not drawing footage at its full resolution when previewing. Set it to Full, and your playback may be a bit slower...but it will look like it should. This does not affect your output--it's a setting for Timeline previews only.