Why won't vauxhall vivaro rev above 2500rpm?

Could be a fault with the air meter on top of the engine....may need replacing. This problem occurs when the engine is cold....the result is it just won't rev enough to get the van up to speed so you end up going along at about 15mph until the engine is hot. Then the van suddenly takes off and is normal. The garage will need to test the van when the engine is cold, and using their hand-held diagnostic computer for Vauxhalls. Only a main dealer is likely to have the diagnostic tool. The new part is not that expensive....its some kind of computerised air meter thing.