First try Pro Tools>Setup>Playback Engine. Start by selecting higher buffer size then try incrementally lowering.
Generally speaking, low buffers are better for recording (lower latency) high buffers are better when editing (less taxing on computer resource).
It might help to give some more info on what outboard equipment you're using and your computer spec's;
Windows users need to provide a Sandra report
- Please - Use SiSoftware's Sandra Lite application to generate a full system profile:
www.sisoftware.co.uk
Under SiSoftware Sandra, click on "Download & Buy" to download the
free Sandra Lite version or purchase other versions. After installing Sandra, click on the 'Hardware' tab and run the 'Computer Overview', then copy and paste into your post here on the DUC.