Was driving my 2010 nissan altima and it just stated running rough and wouldn't go fast, it hesitates and runs very rough. Sound like something grinding and missing. What can be wrong with it?

motor revving - CarGurus

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CarGurusDec 28, 2012 - motor revving - Sometimes the motor revves up and the light go dim on... If in park it takes alot of rev. to make the pop sound. ... 1996 chrysler cirrus change a bad fuel injector and now the engine revs ... Used Nissan Quest".

Idle issues... cold runs great... after warms up it hesitates ...

forums.nicoclub.com/idle-issues-cold-runs-great-after-warms-up-it-hesita...

Feb 20, 2009 - TYC 800107P Nissan Altima Replacement Cabin Air Filter ... when i go to park the car and throw it in park it goes up to like about .... If I leave it in drive when the idling starts to hesitate it tends to make the car jump forward.

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The valve cover gasket has nothing to do with the engine running rough. It simply seals the mating suface between the valve cover and the cylinder head. When bad, it will leak some engine oil. It will NEVER cause your problem. I suggest taking it to a reputable repair facility, and never return to the place that gave you the first diagnosis.

I just looked those codes up and the PO420 states that your Catalytic convertor is bad. If the other code was supposed to be PO300 that states multiple cylinder misfires. It's very possible that the misfires ruined the convertor.

if your chain becomes loose it will become very noisy and should be replaced since most nissans are interference engines which means lots of money for a new engine

You state you changed plugs and wires - Do distributor cap & rotor? If not you should then set timing. That should cure hesitation (might change fuel filter also and run good fuel system cleaner through it)
Prob 2 - sounds like a u-joint or possible brake pad/rotor issue. Does the squeal only happen in mornings when cold? Sometimes after sitting overnight, rotors get condensation and when backing out they'll squeal/grind. Somethings to look at...

My 1994 Altima has always had rough idle but runs smooth. I now have 176,000 miles on it and drive it daily. Best car I ever owned. I wouldn't let the rough idle bother me.

tell your mechanic to look up Service Bulletin #96-032 this could fix your problems

Check the SPARK plug condition, and AIR gap.
Regards,DT

take the car to autozone and have them run a code search on it.

take to dealership , that also is a recall problem with alot of altimas , they need to flush system and check water valve , got mine fixed working great ever since , never hesitate to ask dealership about recalls , they have to fix for free