Do you have a spare analog or logic board for a Mac Plus ? This will be easier to throubleshoot the issue.

As for now it can be a lot of things going from the wire harnass between logic and analog board, faulty flyback, voltages out of specs, bad deflection yoke, faulty resistor, diode, transistor somewhere...

Constantly rebooting often means the voltages are out of specs: what are your readings on the +5v, +12v and +12v line ?

Have a look at this:

http://home.earthlink.net/~gamba2/images/plus_analog.PDF