Jerks like it is stalling when cruising. ODO does not work

[Mother Drives an IQ]

Hello everyone new to this forum. Decided to join because I could not find much on the issues just by googling.  There are two main concerns with my mothers car. First a bit of back story. She bought the car with the CEL on and was told that it needed the EGR valve (previous owner had it looked after by the dealership). I confirmed as much by scanning the vehicle for faults (EGR malfunction code) and removing it for inspection. It was pretty mucky in there.

I ordered the parts for her and did the repairs (I work in a private garage. One thing to note was that I could not get a genuine part and so had to go aftermarket. Once I received the after market part and went to install it I noted the mounting mount was a bit off so I took it to the machine shop where they made a template off of the old part to adjust the holes on the new. I installed it and fitted everything back together and cleared that fault code. When testing the vehicle for I noticed that her odometer wasn't working when I pointed this out to her this was something she did not notice (typical). Anyhow the vehicle was driving great it finally had power and could accelerate (less the odometer not counting the mileage). This was until about after 2 weeks when she said that the CEL, VSC and traction control lights were back on and its jerking.

So I confirmed that it is jerking at low speeds when cruising. Accelerating the thing is fine and moves no problems. I scanned it and now I have two fault codes. In the engine module is  P0420 (Catalyst system below efficiency) and the other is in the Instrument cluster for loss of communication with the instrument cluster. 

To note the speedometer works so we can tell how fast you are going its just the odometer is not counting. with the P0420 I am stuck. Can anyone offer some help? 

[Mother Drives an IQ]

Bump.