My wife came home from a short local journey and said that when stopped at a junction with her foot on the brake the engine revved on it's own a couple of times. The engine now runs very rough and rattles. It was due for an MOT the next day so, suspecting a misfire, I bought a new set of spark plugs, which I didn't fit because you need a special plug socket. They were replaced by the garage prior to the emisssions test but the spark plugs didn't cure the rough running / rattling. They ran diagnostics which did not show any errors. I have checked the coil packs which look good (the diagnostics should flag if they are faulty) i eventually concluded that the only course of action was to take it to the local Toyota franchise because they are the "experts" After 5 hours they phoned to tell me their diagnostics had not shown any errors. i gave them the go ahead to investigate further but said if they aren't sure what is wrong they need to speak to their technical support. As they charge £125 an hour I need to get this fixed quickly. I have looked on the internet for possible causes. The EGR valve and the MAF sensor are suggested but they are both monitored and failure will flag a diagnostic error. As I am sure the problem was caused by the underlined /bold above. I found only 1 thread that mentioned an "engine surge" that suggested the timing chain retainer being loose. Any suggestions as to what is causing this problem would be greatfully received.