So I've been having a strange problem lately...

For some reason, the NAV doesn't think I'm on the road anymore, so if I try to get directions to somewhere, it doesn't think I ever "proceeded to the highlighted route" even though my car is clearly traveling the street highlighted in blue.

I've tried calibrating the GPS to get my car perfectly aligned with the road, powering off the NAV unit so it reloads the system, a different Map disk, but still, no dice.

Anyone have any ideas?

Which version software do you have? If it's 5.1 then:
a) do you see "GPS" sign in the top left corner? This means you have sat. lock, otherwise "dead reckoning" will be used which gets progressively worse.
b) you could go to secret menu and see satellite info, you need at least 3 to calculate the position.
c)Also from the secret menu you could reload the software. Not sure what do you mean powering off NAV system? Did you remove the fuse, or disconnected 12V battery? That's the only way to reload software.
My guess is either software is corrupted and reload should fix it, which in your case it didn't , or GPS antenna is not properly connecting. If you have good satellite fix and reloading software doesn't help, then I would imagine trip to the dealer would be my next step. Hope this helps.

Thanks for the response Pete.

A) Nope I have 6.1, loaded a 7.1 disk.

C) I did reload the software by rebooting the nav system (like my radio stations were gone)

The strange thing is- today it started working out of nowhere!

This is strange, and it's happened before.

What could this intermittent problem be caused by?

There are a lot of things that can degrade gps performance. Occasionally, a satellite fails and has to be replaced. Diagnosing a failed satellite and replacing it takes time, usually at least several weeks, during which the other satellites may not be sufficient to provide good accuracy in some locations. Those system problems are uncommon, though. Receiver and reception problems are far more common.

When you park in or near a metallic structure the satellite receiver looses reception due to the metal between the transmitter and the receiver. Many modern houses have steel framing, metalized sheathing, or metalized insulation that can interfere with reception. It usually takes a few minutes to regain satellite reception after exiting the shadow of the metallic structure. Depending on where you are and where the satellites are at the moment, it often takes longer.

A damaged antenna, kinked antenna cable, or loose antenna connection can also degrade the signal at the receiver and cause poor or no reception. Those are not likely in a late-model car, unless you have been installing things behind the dash.

Finally, the military sometimes degrades the signal intentionally, either for testing or for national security purposes. Because gps receivers can be used to deliver guided weapons, they sometimes degrade the unclassified signal to deny that capability to an enemy.

Well, my house doesn't have any steel framing, plus, this happens on the highway as well...

The only tinkering behind the dash was to attempt to install a backup camera (which didn't quite work out as i had hoped). I guess it's possible the connection is loose... although i did make sure every plug was reseated tightly.

Does anyone know which is the plug for the antenna?

But even if it was a loose connection, It still shows me exactly where I am physically on the road, just doesn't assume I'm on the highlighted route, and won't tell me any directions. The gps is accurate, it's the nav thinking im not on the road.

Seems to me there might have been a TSB (Toyota Technical Service Bulletin) on this exact issue awhile back. Maybe a search of these forums wll provide you the info.

I had the same problem. The GPS showed the position of the vehicle but the NAV unit would be waiting for the vehicle to get on the road before starting. The problem was the internal compass was intermittent and eventually failed. The NAV unit seems to use this to determine that you are going in the right direction before it kicks in.

You can see this if you check the diagnostics screen and look at the sensors. Normally the internal compass will give the proper directions but in this case it was pegged at 0.

It required the NAV unit to be replaced (under warrenty).