Here's a link that may help. The RPM sensor is also known as a crank position sensor. http://www.volvoclub.org.uk/faq/EngineSensors.html#Crank_Postition_Sensor
The engine is not running!
I found that a mechanic that used non Volvo transmission fluid caused this problem in my 2002. Once it was replaced with genuine Volvo transmission fluid it stopped surging.
between 500 and 850 rpm
Code 214 engine speed (RPM) sensor Intermittant signal
850 RPM,
It is the RPM Sensor or sometimes called the Crank Position Sensor.
750-850 non adjustable
The 2015 Volvo XC70's engine produces 240 hp @ 5600 rpm.
The 2011 Volvo C70's engine produces 227 hp @ 5000 rpm.
The 2003 Volvo XC70's engine produces 208 hp @ 5000 rpm.
The 2001 Volvo S80's engine produces 197 hp @ 6000 rpm.