On-board computer showing increased fuel consumption

[radek at pop.istar.ca]

Hello listers:

The on-board computer in my car is acting up.  It shows current fuel consumption, usually
not exceeding 20  liters/100 km.  Lately, it started occasionally showing consumption in
the range of 50
liters/100 km when driving in 5th gear which is absolutely outrageous.  I wonder if it's
the computer that's
gone wild or is my car actually burning all this gasoline.  The problem comes and goes,
I'd be driving on the
highway showing 10 l/100, next time I look it's showing 55 l/100, a few minutes later it's
back to 10l/100.  All
this with no noticeable difference in the way the car behaves (acceleration, noise etc.).
 Where is the computer
getting its data from?  Should I simply disregard the computer or is it a hint to start
digging into the fuel
system?  TIA for any suggestions.

Radek
88 90'Q