a3 8l 1.6 missfire issue

[alex-a3]

hi guys im from Ireland and i bought my first car last week a 99 Audi a3 8l 1.6

it has started to misfire when it reaches temperature
i plugged it into vag reader and i got a revolution sensor fault intermittent ,

other than that it drives fine and im pretty happy with it ,

does anyone on here know how to solve this issue i don't have much of budget and trips to mechanic have to be minimal

 

[alex-a3]

the fault codes i am getting are
00513 missing signal intermittent

00635 lamba

0533 idle intermittent

the idle fault is new it wasnt on the car the last time it was plugged in ,
also i noticed that i have no airbag light in my car ,
anyone know if i can buy 1 anywhere
it was more than likerly taken out by previous owner to hide the fact it stayed on does anyone know how to turn it off ?

really need help on this guys ,