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JimT

Is your emissions systems still intact? - Could be the system trying to compensate for something it detects with one or more sensors. Possibly NOx reading fluctuations causing the ECM to use different amounts of EGR to compensate. Could even be an issue with the EGR system itself. Maybe the EGR valve is sticking at a certain % open/closed.

Could also have an exhaust leak leading to inaccurate readings from some sensors. That causes ECM to compensate when it might not need to, leading to bouncing up and down? Just a thought.

Could also check turbo, make sure it's functioning properly and actuator is also working properly.

Of course you should verify all your sensors are functioning within spec and not providing erratic readings. There's a diagnostic in Insite that can help with that along with using Data Monitor to see what readings the sensors are reporting.

The reading on your dash is usually based on a value supplied by the ECM then additionally manipulated by the OEM systems. For instance my Pete takes the ECM value and averages it out over X amount of time, resulting it smooth up/down transitions beign displayed on the dash instead of quick spikes/drops. I know volvo has their own methods. It could in fact be an OEM/body controller issue or even some kind of communication problem. It's really hard to say.

You can monitor the fuel economy value using Data Monitor in Insite. That will give you a better idea what the ECM is actually seeing/doing compared to what is displayed on the dash. Might be a good starting point to help narrow it down.

Finally, at the end of the day that number is nothing more than a calculated estimate based on many sensor readings and tables in the ECM. It's a best guess. Using a boost gauge and keeping the boost pressure as low as possible and steady will give you much better fuel mileage in the long run.