engine running slggish when cold.ive had anew head gasket fitted with new plugs, distributor, leads and water sensor since being in the garage. ther've adjusted the tappets as well, but it feels sluggish even through improved. now they are saying it could be the cat. which could have been damaged when the car overheated due to the head gasket going.could this be the problem(HELP SOMEONE),ive exhausted all possibilities. can anyone help.