If XM Weather information refresh is erratic, missing information and inconsistent 

Are you getting some weather, but not all: I'm getting lightening but not the weather overlay
The weather packets that are received from XM Weather have time stamps. If the MFD system time is incorrect (off by 4+ minutes), the MFD will process the received weather as stale or old data and will not allow it to display. 

First: On the MFD go to the "Trips" page. Cycle the button labeled "Display" until it shows "Display status"

On the Trips/Status page you can see the signal strength of the weather products received.
If the signal strength is "weak" or "none" this could be an indication of a bad antenna or coax.
If the signal strength is "good" go to the "Aux" page group.
On the "Aux" page select "Setup Time" on the R/H side of the display.

If the "System Time" on the MFD has begun to drift, and needs to be reset regularly, this could be a sign that the CMOS battery inside the MFD has begun to fail; you might consider having the CMOS replaced (work with your avionics shop - not field replaced). To update your 'System Time':

- Make sure your aircraft is outside with a good GPS signal
- Power on your Avionics and go to the Aux Tab, select System Time
- choose Manual; set the date and time based off your smart phone, save
- Power cycle the MFD
- Go back to System Time, set the time to Auto (preferred) or GPS
- Power cycle the MFD, and verify the MFD Data and Time remains correct
- Check your Weather

If your service level and signal quality are both showing none, you must have your shop check the antenna.

If the GPS radios are sending the incorrect date and/or time to the MFD, this will interfere with the weather.
- go to the System Time from the Aux tab
- check all three settings

- use your smart phone to check the UTC time
- if the date, or time is off (+/- 3 minutes), check with the GPS radio manufacturer, or your shop to fix the issue.