[29Cabriolet]

Hopefully someone will have hint on how to troubleshoot this. Sometimes the engine will turn over hard pretty much the way it would with a weak battery. But if that were the problem, it would be all the time. With multiple tries it will finally turn over enough times and quickly enough to start. This has occurred from dead cold starts and hot engine starts where it was shut down anywhere from a couple minutes up to an hour or more. Very sporadic. Occurrence intervals have been anywhere from a couple days to a couple of weeks. Battery is about 3 years old and has a good charge. Cables are relatively new and clean. Starter is not a complete rebuild but had brushes replaced 1-1/2 years ago.
Any ideas would be greatly appreciated!