question about my 2012 hyundai veloster

Hi guys! my boyfriend has a 2012 hyundai veloster that he recently bought off
of a mechanic about 3 months ago, we noticed there were some weird noises
as well as weird jolting movements when coming to a complete stop and when
starting to accelerate. we ran it through a computer at autozone because his
check engine light turned on (this was about 2 weeks after we got the car) and
it came up as a catalytic converter issue. Speaking as my father is a mechanic
and i have been around working on cars and building cars since i was about 11
i knew that problem was pretty useless because my boyfriend was already
planning on taking it out and try to pipe the car. So two days ago him and i were
going to meet up at my dads so we could go grab lunch and as he was turning
into the complex his rpm shot up to 6000 rpm and refused to accelarate, so I
told him to pull over and I checked under the hood tthere was no check engine
light on or any lights for that matter so i told him to rev the engine a little bit and
as he did so I realized that the car actually went into protection mode, i told him
to release the gas and idle the car over to the other side of my neighborhood
and we did so but his car also kept jolting without him touching or pressing
anything. My dad took a look at it as i reset the battery to the car, after it was
done resetting my dad revved the engine again and it sounded normal until it
completely stalled out. We got the car running again and just got it towed to
another mechanic to get a second opinion and he just said the catalytic
converter is clogged and needs to be replaced. The car has now been sitting at
another shop for the past two days but he hasnt really been giving us any
updated so i wanted to see if anyone here has had the same problem and could
possibly give us a better idea.