Intermitent Starting Problem

I don't drive my gold dart everyday so I always keep the battery hooked up to a battery tender. Not always but often if it doesn't hit right off the car won't start. I can crank it over but it doesn't seem to want to fire. Sometimes when I turn the ignition switch back off the engine seems to jump or maybe it fired then. It has done this since I've had it and I have changed out the entire ignition system to the FBO unit with a Mopar performance distributor. The only thing I haven't done is to check the wiring job that was done when it was converted to the electronic system. The engine is a built up 340. I'm running 18 degrees initial with 34 total. The carb is a 750 Quick Fuel.
This happened again yesterday so I put my large wheeled charger on it, set it on 40amps and it fired right up. I'm wondering now if the battery isn't big enough or bad. Any other suggestions??? Thanks.