Momentary loss of current during start

I'm new to this forum so please forgive any lapses in protocol. I purchased a used 99 Expedition with 100K. Recently I notice that when I turn the ignition key it sounds like the battery is drained for a moment but then the starter turns over and the vehicle starts. When this happens the clock resets to 12:00 like my VCR and needs to be reset. With my limited electrical knowledge I am suspecting 1: a battery on it's last legs, 2: An alternator on it's last legs or 3: a Starter on it's last legs. Any electricians wanna help me out? Thanks