ermm.. could be one of many things here..
common problems that can cause similar are..
cam sensor, crank sensor, O2 sensor (you have 2), throttle position sensor, engine temp sensor...... these are just to name a few items.
i am affraid that to diagnose this fault one would really have to see the car
i would normally expect a blocked exhaust to be more noticable at the higher rev range. .
One possible test you could do is to pop down to any local garage and for a few quid get them to do an emmission test on your vehicle and ask them about any strange results..
(erratic readings still show up even tho you have a cat. Plus you test your cat at the same time:)