Clicking trying to crank

[Red Silverado]

This is my first time asking for help. I have a 2002 Suburban with 5.3 and 2wd. This afternoon when I went to crank it all it did was click. Tried several times and did the same. After a while I put jumper cables to it from my Silverado. Then after about the 3rd cranking it started up. I had the battery check 2 times and both test good. Been noticing that when I go to crank for some time that it was getting slower. Also all of this happen on a hill with the front downward (don't know if that matters). Was told possible a dead spot in the starter. Also have cranked 4 times since with no problems. Any suggestions?

 

 

 

Thanks

[AirForceAngler]

I'd say either your battery is going dead or you have some corrosion in the cables.