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Look for this link near the top of this(and every) DUC page. Its nearly impossible to offer any advice with zero info on the setup. A "new PC" means nothing, especially if its incompatible with Pro Tools. Here's the info we need:
1-A Sandra report on the PC?
2-what interface?
3-what exact version of PT?
4-what plugins are used in the session
5-what 3rd party plugins are installed, even if they are not used in a session, some can cause issues. The way to test for this is to drag 3rd party plugins out of the plugins folder(I usually make a new folder on the desktop and move plugins into that folder. When I know those plugins are not the issue, I simply move them back into the plugins folder). On windows, plugins are here: C:/Program Files/Common Files/Avid/Audio
BTW, make sure that(in Preferences) you have auto-backup enabled(I set mine to save every 1 minute and keep the last 15 backups). Opening the most recent backup file might show the audio that you think is missing