P0420 running rich

Hi everyone 

I would greatly appreciate some help: 

The p0420 comes on after a few days of driving following reset. - with the new thermostat;

Before I replaced the thermostat (was opening too early) the code would come on much quicker - in the first hour of driving. 

The car is running rich - black smoke + raw fuel smell from exhaus + carbon deposit on the pipes.

The cat and 02 sensor were replaced a couple of years ago by a shop wallowing theft attempt.

The o2 sensor stays at around 0.68v, looking at videos online for narrowband after cat o2 sensors it seems to be normal ? 

The short and long term fuel trims are close to 0 ( one journey they were at 0, another 0.8, another 2.3%)

When I've replaced the maf sensor the first replacement caused the short term fuel trim to go -23% at tickover. Second replacement stays at about 0.

I've replaced the o2 sensor (aftermarket)- no change-  either they are both good or both faulty in a similar way !?

I've removed a vacuum hose as a test and the short term trim + o2 voltage changed to + 25 ish, plugged it back in and it went the other way then settled around 0 (as I would expect) 

At tickover the car runs almost electric motor like but not perfect- there is the occasional drop but a slight one

It has power but feels a bit "wavy" under 2000 rpm under moderate load 

Any thoughts/ suggestions will be greatly appreciated !!