I think you have a bad connection, even though you've checked the grounds and battery connections, the heavy power cables should be ran directly to the engine ground included, with the starter solonoid in series with the positive power lead. Another possibility is the starter is not engaging the flywheel, either due to worn teeth or the starter drive gear, there is a set of switch contacts inside the hump on the starter that starts it spinning when it is engaged, and the starter solonoid could also have a problem. Since it is intermittent, it is hard to find, it would be ideal if you had a second person to try to start the car when the problem is occurring and you could use a volt meter to find where the voltage is either missing (electrical problem) or it the voltage is getting to the starter, then it's a mechanical problem I described above. Also if the wire gauges are too small in the batter relocate, it will have too much voltage drop, and be more susceptible to this problem. Post your results. Good Luck.