[models 760-779] spark is not there sometimes, changed plug still won't start, battery good, cranks over but no spark at times.

Hi,
If it is intermittent, it is most likey the ignition switch. The bad thing to is not easy to isolate the engine with most recent models; the plug that is the ignition ground operates the anti-backfire solenoid also (generally white rectangular plug near the dipstick, usually white and gray wires.) You can unplug it to eliminate the rest of the machine and check for fire, but you would have to make a temporary coonection some other way to open the solenoid.
Peace,
Clarke