You need to check the IRS monitor function. On the ground only you will see the maintance function on the CDU options page. (init ref should take you to it). Line select the maintance fucntion and then line select IRS monitor. This will show the IRS drift rates for the flight just flown. I believe the limit is now 2.5nm/hr.
This check is normally done by the crew after ETOP's flights.
There is definitely a fault if the EICAS is flaging up when you turn the GPS off. Are you sure your in a good area of radio updating when you turn it off ?
p.s rudderrat, the IRS's always drift with or without radio updating. The updating is for FMC position not IRS position. They are 2 seperate things.