timing error -timer setup

Hi there, I am very new in programming micro.I am using stm32f100 discovery board and made a small code to turn the stm board led on for 5 second once hit the user push button.then the led goes off.The partial code below I have to do the work.The program runs my led on for 5 sec only the first time I hit the user button.But next time I hit user it gives me 4 sec, and keep changing everytime to 3s,4s,,like random.I could not figure out what I am doing wrong.Could anyone please help me on this.Thanks in advance.

TIM3->PSC = 23999;         // Set prescaler to 24 000 (PSC + 1)
TIM3->ARR = 5000;           // Auto reload value 1000

while(1)
{
    if(GPIOA->IDR & (1 << 0)) //if user ush button is set
           {  TIM3->CR1 = TIM_CR1_CEN;//enable timer3
              TIM3->SR &= ~TIM_SR_UIF;//  clear uif bit
        while (1)
                     {
                                 GPIOC->BSRR = (1 << 8);       //until UIF flag is not set led 4(pc8) in stm board blinks

                         if(TIM3->SR & TIM_SR_UIF) // if UIF flag is set
                                 {
                                                        GPIOC->BRR = (1 << 8); //clear the pc8
                                                        break;
                                 }

        }

    }

}