There are some cars that have windows which interfere with the signal. If you move the mount to the side towards the side windows you may have better luck or if you take the unit out of the mount and hold it closer to a side window it may sync (though only temporarily while at that position).

What kind of car do you have it mounted in?

This thread deals with the issue:

http://wi-geocaching.com/modules.php?name=Forums&file=viewtopic&t=5303&start=0

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The Silverado does not appear on the list of vehicles that are known to cause issues with the I-PASS transponders (and therefore are likely to not work well with GPSrs either). (Although they do say that all ’99 Chevrolets with Solar Ray/Solar Tint windshields can have an issue.)

I-PASS Special Windshield Mounting Instructions

Are there any sources of interference in the truck? Maybe a CB radio, a high powered car stereo, external cell phone antenna, etc.?

Maybe the unit is defective and the vibration when sitting on the dash causes it to fail?

Any chance you can try it out in another vehicle?

Sounds like an intermittent problem that may be a pain to figure out…

Do you have the latest firmware updates?

@Team Outdoorsman wrote:

the latest what????

So you don’t have the latest firmware then… 🙂

(Firmware (n): computer programs contained permanently in a hardware device (as a read-only memory). Garmin uses the terminology “unit software”. Where I work it is usually called “embedded software”.)

Go here and download the Garmin WebUpdater software:

http://www8.garmin.com/products/webupdater/howtoinstall.jsp

Basically, this program will check for the latest firmware released by Garmin for your GPS and update it if it is out of date. Even if this doesn’t fix the problem you are having in the truck, it is good to be running the latest firmware so you don’t run into some other weird bugs that they have fixed since your unit was manufactured.