Thanks to all of you who helped with advice.
The car was hesitant, when I put my foot down it wanted to stop, no power, the fuel pump was making a noise.  All this after I changed the Oxigen sensor.  
The first thing I did was to change the fuel filter.
I went for a ride and all was fine.  The filter was clogged.  Lucky that it was not something more serious.
Zoltan