Sounds like maybe some clips are actually 30.00fps while others are 29.97. You can try changing the
Use drop frame code setting on the General Preferences tab to see if that solves the issue.
You can check each clip's frame rate by using MediaInfo, and now they have a new
webpage that will automatically give you a text list of all technical details. No need to download and install the app at all!
If you do have different framerates, pick the framte rate that the fewest clips are recorded in and convert them using Handbrake or VirtualDub 2 to the other rate.
The easiest way to replace the existing clips in your project would be to rename all the changed clips and then copy the transcoded clips to the same folder. Give them the original clips' names, and when PD opens up your project it will automatically load the new clips.