Likely the cause is related to something you installed or did recently. Without a little more information it is difficult to troubleshoot your problem. I would suggest you update your graphics card to the latest driver, run some basic CPU diagnostics, test your drive throughput, disable virus protection and try again.
I hope this helps,
I have been using SD MP4 files as source with great results.
From what source? There are no standard camera formats shooting in SD using MPEG4. And Premiere is designed to work with standard camera formats. Video files that deviate from those standards may or may not work well.
Work through all of the steps (ideas) listed at http://ppro.wikia.com/wiki/Troubleshooting
If your problem isn't fixed after you follow all of the steps, report back with ALL OF THE DETAILS asked for in the FINALLY section, the questions at the end of the troubleshooting link