In two separate occasions the fuel gauge read completely empty and warning light on when starting after parked for more than 1 hour lunch. Ambient temperature were 60-85F and fuel reading prior to shutdown were between 1/4 and 5/8. Car parked on fairly leveled parking lot.
After driven about 5 minutes on the road the needle suddenly jumped back to previous reading. There is no sign of engine abnormality.
I've checked gas cap in both occasions. No sign of leak or looseness.
Any suggestion?

Just a guess -- maybe a sticking sending unit in the tank?

Just a guess -- maybe a sticking sending unit in the tank?
That would be my guess too. Actually, it seems kind of obvious to me that this is exactly what's happening. If it were the gauge sticking, the low fuel light wouldn't come on too.

I don't see a sending unit in the tank but a complete fuel pump assembly at $600+!

It's there... the site I checked doesn't show the part number, but it lists for $89.96 and sells for $70.30. #17 in this diagram:
http://www.lithiatoyotaparts.com/schematics/8/8512735.gif