Cummins ism

good day all I have a 2004 international 8600 single axel with a Cummins ism egr 330hp and 3.91 rear end ratio

Issue I’m having is truck has no power very sluggish. To me it feels as if the turbo actuator is confused as if it doesn’t know what it wants to do with the turbo vanes to keep them open or closed. Whiles pulling a load yesterday the check engine light came on the exhaust got very loud and truck had a sudden increase in power aswell as I noticed the rpm swung to 2100 instead of the 1800 I knew it to rev to. Any ideas on what can cause these issues with a Cummins ism this is my first time dealing with a Cummins ism