2004 Vauxhall Vectra engine not revving properly

driving my car the other day it seemed that when i accelerated it was missing.the engine management light came on so went to garage and was told my cat was blocked.new cat on and same problem.new fuel filter,same problem.diagnostic shows nothing .when you rev the engine its missing around 4000rpm .when you rev slowly its seems ok . any ideas ?,my mechanic seems baffled and the vauxhall garage have not seen this problem before .mechanic says its like the exhaust fumes are not escaping . HELP !