Renewed EGR valve with after market part now kangooring slightly?????????

[malibu2007]

hello im new to the forum although had many VWs , my problem is that the EPC light on the dash has been for about a year , i had my timing belt renewed when i first got the car and the garage nackared it up so i had to take it to a friend of mine who put new lifters in etc etc and new timing kit . I cant remeber if the EPC light was already on so i just put it down to him not having access to diagnostic equipment and the engine being in bits so he couldnt reset the light .
the car has drove perfect since giving me 40mpg over the first few months of him doing it (vw bora 1.6 16v 2002) .
over the last 6 months the MPG has dropped to more like in the 20's so i bought a VAG KKL cable of ebay downloaded the free ross tech VAGCOM and the german CARPORT . the free rosstech wouldnt give me anything but the carport came up with EGR valve circuit too low EGR valve circuit too high , also P0571 brake switch circuit A...
I did the diagnostic check first thing in the morning car was freeing cold ive read somewhere that if you dont warm the engine up first then it can tell you that the EGR valve is not working is this true???? i didnt read this before hand....
anyways so i orderd a after market EGR valve off ebay as the mpg was getting low and sometimes at lights there would be slight erratic idle , so i just thought its egr valve thats gone and its a common fault and ive replaced loads of these at work..
the EGR valve i took is genuine and now the car sometimes kangroos when im cruising at aroun 30-40mph and the EPC light is still on the dash , and ive plugged the laptop back in to have another check on CARPORT this was after a hours worth of driving now it says p0571 brake light switch circuit A , so gunna order one of them but all my brake lights still work so how can this be the fault?
should i put my old genuine vw EGR valve back on to stop the kangrooing? how do i test it off the car ??

[itavaltalainen]

Use VCDS lite and this will allow you to do output tests. Brake light switch won't have anything to do performance.
Also worth looking at measuring blocks to see if all signals from various senders, sensors and switches are OK.

[Doctle Odd]

Changing the brake light switch is your first priority and buy a genuine one, the switch is also connected to the ECU. then clear the codes and drive the car. If the EGR you bought is faulty it has 12 months guarantee VAG cars don't like spurious parts much but change the switch ASAP