engine problems

hi everyone...im new here and i was wondering if anyone can help. i have a 1995 colour concept golf gti 2.0l 8v.it has a scorpion back box fitted and a pipercross induction kit.the problem is that three days ago it started idling very badly.i also noticed that it loses power at 5000rpm,almost like a limiter. i had a guy put it on a fault code reader and it threw back 2 faults..1 crankshaft position sensor not working correctly and 2. air reading is out of range,something to do with the lambda sensor. i notice when driving from cold there doesnt seem to be problem but when the car heats up it runs badly. ive changed the distributor cap and rotor arm but to no avail.can anyone help?