Re the batteries, have had a similar experience. Have a garage queen (928) that I keep on a trickle charger, and only take out for local runs of 20-30kms. One day I happened to do a run, park, then out again to pick up g'daughter from school , then park - flat as can be. Got it jump started and home. Did a test at next opportunity - put a DVM on the battery while it idled . All good , showing 14.1v (fully charged battery)...for 20 mins, then the volts fell right off - alternator failing once thoroughly hot. With a new alternator the 6 year old battery is doing fine. 928s having a large complicated electric system (32 fuses and more relays) an annual electrical maintenance is recommended - clean and deoxit all fuses, pull relays and reseat, undo and clean all ground connections. So, with any little used car I recommend something similar, especially the ground connections - undo, clean with something abrasive, apply Deoxit, refit. With a ground cable that is shrink wrapped they can develop whats known as black wire corrosion inside the wrapping, so they need checking, and occasional replacement.