Warm climate, summer oil, clean terminals and connections, new battery. Doesn't leave much left except the starter. Sounds like it could be the starter bearings (bushings), armature, brushes, or field windings. My guess would be the bushings or brushes. Pull the starter and check for slop, mainly at the bendix end where it would engage the flywheel. If the bushing is bad it will make the starter armature bind when trying to crank. If the bushing seems fine check the brushes and where they contact the armature. Armature can be cleaned up with fine emory cloth and replace the brushes if you feel confident doing so. If you can't replace this yourself or find anything else wrong take it to a starter / alternator shop who can test and / or rebuild it for you. Rebuilding will be far less than the cost a new one.