My 91 caprice has a new motor in but it jerks when it warms up and sometimes it shuts off

Make sure you don't have any fuel lines close to the exhaust or anything hot. Outside of that this will require a hands on inspection as there are many variables on this type of car that can result in poor running when hot. If you don't work on your own cars then use this link to find a good shop near you. https://repairpal.com/repair-shops Alex

ignition modules on the tbi and L-series engines are a common failure they get hot and die. zomstimes work again when cool off and sometimes dont.