Or do you mean the IMAGE on the screen is wiggling?
If it is moving back and forth about once a second, look to see if anything with a magnetic field is near the monitor. This could be the computer's power supply, an AC adaptor "wall wart", or even an electric fan. One time I had two computers next to each other, and their monitors interacted to produce wiggling screens on both.
The stray magnetic field at 60 Hz is competing with the magnetic fields of the deflection coils in moving the electron beam. If you can't remove the offending field, stick a sheet of steel between the source and the monitor. A steel cookie sheet works nicely.
If it is jerking, look for one of two things:
- Bad connection to computer.
- Dirt inside the monitor causing the high voiltage to arc.