|By 'd/c port' do you mean the charging jack on a laptop or is it some piece of hardware I haven't heard of? And the problem started just after you got it back?|
If it's just a new charging jack then the shop had no reason to install any software or or make any hardware changes. But you can run MSCONFIG and click the 'startup' tab and see if you can tell if anything has been added. You can run a simple malware scan with something like malwarebytes:
and see if it picks up anything.
You can go back to the shop, tell them what's happening and see I they'll admit to doing something that may have caused it. Of course they'll say they didn't do anything but replace the port and have no idea what's going on.
Or it could be slowing down to conserve power and reduce heat: