04 Buick LeSabre does when put into gear...

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Problem started 2 weeks ago with a clogged catalytic converter. Catalytic converter omitted and exhaust fixed. We then had to replace the quick connect from the fuel line to the engine where it was leaking. Now car will start and go in gear but wants to die when you try to move it, did not have this issue until the catalytic converter issue. Please help, this is our only vehicle. Thanks in advance! Car did not have any issues before 2 weeks ago