Probably the worst case scenario is that your engine has overheated drastically and locked up while running. Then, when cooled off it cranks. My Dad's motor home (BigBlock Chevy) had a pinhole in a heater hose and it lost ALL the coolant while driving at highway speed. It ran a while and finally locked up but got hot enough while still running to ignite the plug wires and melt the plugs. He refused to believe what I told him had happened and he changed the wires and plugs and drove it again til lockup(with another onboard fire involving plug wires). When he called me for help, I found and fixed the leak and filled the cooling system with water (almost 9 gallons!) By now the compression was so low it would not start but cranked vigorously. Check your cooling system and put a shot of oil in each cylinder to see if that affects the cranking speed by sealing the rings. I hope this is not your case but it's simple to check and might keep you from throwing money down a hole if it proves to be true. Also I believe a lean condition would cause abnormally hot exhaust. Good luck!