Car runs smooth until it hits 35-40 MPH then jerks loses power

Hello, it's definitely hard to tell via a computer screen, but I would say that my mind goes to thinking about what happens when you hit that speed. The first thing I think of is engagement of the torque converter at the transmission. If there is an issue with the torque converter clutch or automatic transmission, this could cause some issues. This is only a guess though! I do lean against an alternator issue, and although computer processor programming could influence things, I would lean towards a mechanical issue.