I have a 2003 GM LT-1 engine in an old body, that's had different intermittent starting problems for the last 6 years. All have gotten progressively worse. Problems are listed old to new.
Occasionally when key is turned to start I will hear a click and then it will start. Other times, it will click and I'll have to release the key and try again. Sometimes it starts right up after that, and sometimes it may take 2-5 tries then start just fine.
Sometimes it will go a week and start great every time. Then the car got to where it would start just fine, but the next time it would click and the starter would turn VERY slowly as if battery was just about gone. After trying again (maybe several times), it will generally start just fine. Lately the car has gotten to where it takes more than 1 or 2 times to start up though: last week it took 19 tries, then quit even making the clicking sound. I left it for an hour, went back, and it started just fine.
I've noticed that the alternator gauge in the dash will go down to 12 Volts or a little less, then go up to maybe 12.5 volts. Lately I started watching the gauge at start up, and it shows above 13.0! Later that reading will go down to 12.5 or even 12.0. It only reads 13V or above only during the first 10 minutes or so of driving.
I recently changed the battery to a new one with 900 cca & 750 regular amps. Did not help situation at all. It has never refused to start (except once having to wait an hour) but I am getting uneasy about driving it & stopping, & then not start I I am stranded. One shop diagnosed this as miss-adjusted neutral switch. Adjusting it helped for a short time, but then back to the same old ways.
Most times the car starts well first thing in the morning.