2000 impala no start condition sometimes

sometimes no crank ??? When you turn the key to start nothing happens ? Starter doesn't engage an spin the engine ? Or the starter does engage an spin the engine but doesn't start an run ?

Won't start how ? The engine spins over normal but won't start an run ? Or when you turn the key the starter doesn't spin the engine over ?

Won't start how ? Does the engine crank when the key is turned to the start position ? Engine cranks no start ! Or nothing happens when the key is turned to the start position ? that is a no crank no start condition . Very different diagnostic procedures . Is there a padlock car symbol lit on the instrument cluster ? Anti-theft system problem !

This may be related to the antitheft system The system is known for turning off the starter and/or injectors when it fails. Is there a light on the dash that says SECURITY? This is a common problem worth looking into. http://www.diy-gm-security.com/

Won't start how ? When your the key to the start position what happens ? The engine spins but won't start an run . this a crank no start condition . If nothing happens when you turn the key to start ,this is a no crank no start condition . Need to be clear on this so we know what to have check . If nothing happens when turning the key to start position ,try moving gear selector to neutral - park neutral safety switch . If when you turn the key to start an the engine does spin but doesn't start an run ,check for spark ,fuel pressure . see what's missing .

Why do so many people ask about an Alternator ?

It has nothing to do with the starting system

You said you have to play with the key & ignition switch
that may be a place to start

if the starter spins but does not engage flywheel. then its the starter solinoid thats bad. its on top of starter, when key is turned to start, the solinoid is activated, and it pushes the gears out to make contact with flywheel to start vehicle. so the solinoid just spins with out kicking out to flywheel. change solinoid or get another starter. have a good day !

Sounds like a "dead spot" in the starter solenoid. Not sure if you will have to replace the entire starter assembly or just the solenoid. Some GM starters have a solenoid that can't be replaced seperatly.
A trip or call to your local parts store (O'Reilly's / Advance Auto) might be helpful.. explain your problem and see what they think.

Does the starter kick out and spin normally out of the car? Have you checked the condition of the flywheel? Ususally the starter doesnt turn until the solenoid kicks the gear out to engage the flywheel. Check if the gear on the starter is bad, it should slip one way, and not the other.

you got a bad ground wire or connection clean it.            Battery wire  ground clean it not on the battery post but the frame.