My actual guess - devoid of any other info - is actually going to be an EGR system issue. So, now that I’ve said so, kboyd, find the vacuum line to your EGR valve. Pull it off & plug it (usually about a golf tee size) and cap the valve. See if that makes a difference. (This will turn on a check engine light & set an EGR code & is not permanent. It will just tell you if the EGR system is the problem).