N42 timing problems

hi everyone iam new to the site just seeking some advice. i have a 2001 316 ti e46 n42 b18 valvetronic bmw i've just replaced the cylinder head but keep noticing after setting the timing whenever i start the engine it begines running lumpy when i recheck the timing the intake cam has jumped a couple of teeth ,i also noticed you can pull the intake camshaft back into timed position by just putting the tool to the rear of the cam and pulling it back into position. this continues to happen each time the engine is started .any advice please.