Which Suzuki 250 do you have? Their current line-up has five different 250cc models.
Since Suzuki’s line-up seems to be fuel injected, you can probably rule out carburetor problems. I’d check the valves to see if they are out of adjustment. I’d also check the fuel valve to make sure it is operating properly. That kind of jerking sounds like what happens when you run out of fuel and have to switch to the reserve, so I hope your mechanic has confirmed there is gas in the tank.
I also think you should take it to a different mechanic. The one you have is throwing parts at the problem instead of doing real diagnostic work. I know it’s not easy, but you either need to keep looking for a mechanic who knows what he is doing or learn to be your own mechanic.
Check all of the electrical connections to make sure nothing is lose, including the spark plug wires. Follow every wire from where it begins to where it ends, focusing on connections that are exposed to road debris.
This could also be a faulty engine cut-off switch, but that seems unlikely on a bike that is only a few years old. Do you store this bike outside, or in a garage?