Flickering when reading data from GDDRAM

Hello, everyone:
I am interfacing the LCD with a MSP430 from Texas Instruments, system 8080 8bit mode, 25K color. The clock frequency generated by the MCU is 16MHz. Initialization on SSD1289 does well and I am able to write data to the GDDRAM. However, I have a problem when trying to continuously read data from GDDRAM, the simple code is as follow:

unsigned char data;

LCD_CMD(0x22);
while(1)
{
data = LCD_READ();
}

void LCD_CMD(unsigned char command)    // function write cmd
{
PxDIR = 0xFF       // Px  is set to output data
CD_LOW();
RD_HIGH();
WR_LOW();
CS_LOW();
PxOUT = command;
CS_HIGH();
}

unsigned char LCD_READ() // function read data
{
unsigned char r;
PxDIR = 0x00;      // Px is set to input data
CD_HIGH();
WR_HIGH();
RD_LOW();
CS_LOW();
r = PxIN;         
CS_HIGH();
return r;
}

The screen keeps randomly flickering every one or two second. The flickering does not take the whole screen. Sometimes it takes half, sometimes quarter. I guess it's because somehow conflicts take place when SSD1289 is trying both to return the data stored in GDDRAM to MCU and to read data in GDDRAM to update the screen.

Anyone has any suggestion to solve the problem ?
If neccessary I can post my init code.