It could be that your starter is going out. If there are no other variables, that's it.
Your car is old, and the Fiesta is really susceptible to electrical issues from corroded grounds, wires, and old batteries.
If you're able, go through the engine bay and make sure that all your wires and grounds are clean. Check wires and connectors.
A loose wire or a dirty ground can cause all kinds of havoc.
If you can see that everything is reasonably OK, then don't replace the starter until it no longer wants to start the car.
Older car, lots of miles, lots of starts. Just a normal wear part.