If you are not successful finding the problem - a cheap and likely cause
could be the major grounds for the all the sensors...doesn't cost anything
other than time...check / clean those grounds - the major ground harnesses
in the engine compartment and the one beside the distributor which grounds
the O2 sensor.  A little air moisture in the air might be playing a some
havoc.  (I'm talking 2.1 digifant here not sure what the set up is on the
1.9)

It worked for me several weeks back.

BT