2002 Chevy Tahoe returned from the dead

Wife complained truck would not start. Opened up the door and noticed that the dome light was on so I assumed the battery was good. Turned the key and got nothing but a bunch of clicks from under the hood. After another attempt at starting it the cooling fan started up and turned on and off several times and continued after I removed the key. Tried starting it again and it started. When I returned home from work that evening I figured that I would check it out again. This time when I opened the door not one light came on. First attempt to start the truck was not successful. Each time I turned to key to the 'on' position something would start to work. First the 'service engine soon' light, then the a/c fans, dome light, then finally everything came on. I tried to start it a second time and it started but idled very rough. After about a minute it was running smoothly. Turned it off and it started up no problem. It does have a half second starter lag when first cranked but always starts. About a month ago I got a knock sensor warning but has gone away after filling up with premium.