Bad idling problem

I have an Audi A3 1.8t Sport and it has had this problem nearly from when I bought it a year ago but never this bad, basically it seems to always idle at around 800 revs and most of the time the engine would sound fine, but occasionally it would drop lower and would start running really bad as if its about to stall. I notice that it would mostly be after a long journey (and hour or two of driving) when it was at its worst. I did get a mechanic out to come look at it and he got 2 fault codes come up(P1297 Throttle body Fault, P1340 Cam sensor & camshaft sensor)but he told me that it was getting that because the previous owner put a cone filter on which he advised me to change back to an original filter which I did. and for about a month the car ran fine I did still notice it would idle at 800 but without stuttering at all. but then I started to notice the problem again after the engine management light came back on. I haven't got the mechanic out again to see if it has the same fault code or not but I don't exactly have allot of money to keep bringing him out to look at it. as I said earlier I did notice when I bought the car there was a slight juddering at idle but barely noticeable and it only did it once or twice. and in the past few day when I have started the car in the morning just after I turn the key I would get three beeps which I looked up and it said it should stop driving my car straight away but I the beeps where coming from behind me and not from the dashboard which some people were saying the beep should come from. I have stopped using the car since then but need it sorted soon. I was just wondering if anyone knows what the problem might be and possible fixes for it as I don't partially want to take it to a garage and spend a ton of money that I don't have to fix the problem