This may sound silly, but when should you change gear on these vehicles? We have just got a Hymer motorhome and the Fiat 2.5Td engine seems fine, except when we push up hills the enngine starts 'missing' and we have to change gear. Is there a governor on the gear ratios? We have read all about the 5th gear problems with these gear problems, but no-one has mentioned this! Thanks from newbees!