2003 Volvo S60 sudden RPM spike from 2K to 3K during low miles driving

volvo
rpm
s60

#1

I have a 2003 Volvo S60 and I experience sudden RPM spike from 2K to 3K during low miles driving. It doesn’t happen all the time, usually once a day. I took to the dealership and they could find issues and no warning light are on. Any idea why?