Sounds like you need a new mechanic. Over head cam engines don't have lifters, that I know of. The engine is a little finicky about precise valve adjustment, and if they aren't evenly adjusted to the correct gap with the engine stone cold, you can get a rough idle. Did you replace the head bolts? They have a lot of tension on them when installed and should be replaced when removing the head. How are your injectors? Do you have a Bentley manual? Even if you don't do the maintenance your self, I would have one just for the knowledge so you are less likely to get snowed...again.