Low fuel light but not low on fuel. . . .

[Red Octopus]

This just started on my way to Ohio this weekend. Me and my '99 XJ stopped for a Mocha and when I started up to go back on the road the low fuel light came on - I PANICKED! - I mean when I stopped I had half a tank and had only gone 238 miles - so I fueled up and it only took 10+ gallons and the light went out.
Now I'm home and 800+/- miles later it did it again with the same symptoms - this time all I did was stop and restart the engine - Wha-La! 'no light.
So whats up? . . . . any ideas ?
. . . . and no the Mocha was for me the XJ does not drink coffee.