2001 Chevy Blazer COLD STARTING

Engine problem
2001 Chevy Blazer V6 Four Wheel Drive Automatic 91,000 miles

HI. MY BLAZER SEEMS TO GO INTO IT'S OWN WORLD. IT WON'T START AND / OR DOES A LOT OF COUGHING [ ABOUT 5 MIN ] AND EVENTUALLY STARTS WITH A LOT OF TROUBLE. I HAVE A HIGH AMP / HIGH CRANK HR. 1 YR. OLD BATTERY AND A 2 MONTH OLD ALTERNATOR. THE VOLTAGE METER DROPS DOWN TO 12V OCCASIONALLY WHEN THERES A BIG LOAD ON THE ELECT. SYSTEM BUT DOES NOT STALL ANYMORE [ SINCE THE NEW BATT. AND ALT. ] IT DOES THIS ABUT ONCE EVERY 8 -12 WEEKS [ NEVER ABOVE 45 DEGREES THOUGH ] AND IT ALMOST ALLWAYS STARTS IN VERY COLD 0 -25 DEGREES... ANY HELP WILL BE APPRECIATED IMMENSLY - THANK YOU