stray voltage on a few pins

hello,

in shown simple code, when read voltage , print serial,  it fluctuates between high , low, on it's own.
i have wire going 5v out on arduino, switch,  to digital pin 2, set input....
i'm finding voltage fluctuates alot, , stays high few seconds after re opening switch.  sometimes if take wire out of digital pin 2, , put fingers near pin, pick high... making project frustrating... i'm using ethernet shield on arduino.  
any special config need clean pins up?  
same set works great on digital pin 4.. on pin 2 , few others these crazy readings.

code: [select]


#include <spi.h>
int light;

void setup() {
  // put setup code here, run once:

  pinmode (2, input);

  
  serial.begin(9600);

}

void loop() {
  
  // put main code here, run repeatedly:
  
light = digitalread(2);
  serial.println(light);
  delay(1000);
}



make simple changes keep pin voltage level 'floating:

code: [select]


byte light;

void setup() {
 // put setup code here, run once:

  pinmode (2, input_pullup);

serial.begin(9600);

}

void loop() {

  // put main code here, run repeatedly:

light = digitalread(2);
serial.println(light);
delay(1000);
}

wire pin's button connect gnd when pressed.

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