Hi,
what means RECEIVER_WARNING... Is there a description for that error? Can you verify maybe with hyperterminal that your longitude value is coming from the receiver periodically.
When you say time, date and latitude is committed properly via the blocks, how does the longitude-values looks like that you mentioned that there is something wrong with it.

When i study this article http://en.wikipedia.org/wiki/Global_Positioning_Systemi see that three satellites should be enough to get the long and lat. So please post here the ascii-string coming from your device.
regards