1998 Mercury Sable Engine Starting

My wife called me saying see couldn't start the car.
I met her and jump started the car successful.
I charged the battery and tried starting the car, which cranked very slowly and then not enough power to start and then the clicking sound.

I purchase a new battery and installed it.
At first attempt it cranked slow, then perfect starts for several days.
Today, I went to start the car, slow cranking, 2nd attempt even slower and 3rd attempt clicking.

I waited roughly 20 minutes and tried again; perfect strong start.

I checked battery, fully charge.
Could it be the starter... or something more complex.