1997 Vauxhall Vectra Car Stalling and also jerks

Hi I have a Vauxall Vectra 1.8 16V Artic It stalls at the signal lights and whenever the spped comes dwon at round about or traffic and than starts do the same again. It happens when the car has covered around 20 miles. some time it wont start unless you wait for 1 hours

2nd problem is it jerks on the 3rd gear ( 20 - 30 mph) and the spped come totally down to 10 and it struggles on pushing the gas make it more jerky and after few minutes its back to normal I have notices this on uphill some time

please advise