Are you switching the IRS knobs from OFF to ALIGN then to NAV? If you are, then don't. Just turn them from OFF to ON.

Turning them to ALIGN while you enter the GPS co-ordinates, then to NAV causes me problems too, whereas turning them from OFF to NAV, then entering the GPS co-ordinates, works fine.

A quote from the manual (‘767 Captain’ FLIGHT MANUAL Part II – Aircraft Systems (cs767_manual2.pdf) page 10):

Quote:

IRS Alignment An IRS must be aligned before it can enter NAV mode. Rotating the IRS mode selector from OFF to NAV begins the IRS alignment. The IRS performs a short power test, during which the ON DC light illuminates. When the ON DC extinguishes the ALIGN light illuminates. Alignment requires approximately 5 minutes.

What I underlined says it all. And it only takes about 30 seconds in FSX, unlike the 5 minutes it takes in real life! ;)

Hope this helps.