1999 Nissan Sentra Inconsistent Starting Car

I have a nissan Sentra 1999 1.6 L thats been giving me problems lately while starting the car. Normally when I turn the key it would crank just for a second and the engine would turn on perfectly, but starting a few a days ago, it was having a different sound when turning on. The crank sound was slower, and took longer to turn the car on. But then it would be normal again the next day. The following day it cranked so slow it didnt turn the car on. And when I tried turned the key again, it made no sound. I just heard the switch noise when I twist the key, I think they call it the relay. My headlights worked perfectly no dimming, I check the battery connections, they are bright silver, I just had them cleaned, so im sure its not the battery. I tried a million times to turn the car on but I couldn't, no crank sound at all. I had the car towed to my house the same night, when I got it home I tried to turn the car on just for the heck of it, and the engine magically started, and the crank sound was normal!

The next day I was puzzled, so I experimented. I turned the car on first thing in the morning, crank sound was normal. Then I turned off the engine and turned the key again, crank sounded slower. Then I repeated this again, crank sound was slower, rougher start. Then I turned the car off and then turned the key again, just to crank a little, but not fully turn the car on, and it just sounded so nasty and harsh, then I turned the key a little again and no crank, no sound, just the relay sound.

Any thoughts?