Four digit seven segment blinking problem

Hi, I'm writing display functions that will display an unsigned integer or a float on a four digit seven segment display with the MSP430 launchpad and I'm running into a problem. 

In the infinite 'for' loop, displaying a number works fine if it's only display one number:

int counter = 100;
for(;;) {
    displayUint(counter); //a function in the code
}

However, if counter is to increased, the first digit of the 4 digit remains brightly lit while other digits blinks very quickly.

int counter = 100;
for(;;) {
    displayUint(counter++);
    delay(1000); //delay 1 second
}

Schematic of circuit: http://i40.tinypic.com/2cqcim0.jpg

Code: http://pastebin.com/LCrHeDur

In this case, I didn't use transistors. I don't get the point of using them and using resistors are simple enough. As for display functions, I'm sure there are more efficient ways of getting individual digits but I haven't found much. If you do know a way, please tell me. Other than that, I'm stuck with the blinking problem and I can't work out why it's not working.