vauxhall vectra 1.8 2002 model?

hi i have vauxhall vectra 1.8 runs ok on dual carriage ways when it comes to running around the streets dies away on excelration there is a thump and then power comes back dont get no warning lights coming on but u know when its going to do feel it through the pedal had it in loads of garages but can not find fault had it on computers no codes shown ive changed all sensors and throttle body and coil pack makes no diff still the same

caroline
May 2011