Xorshift pseudorandom number generator with shiftvalues in progmem

I started implementing a xor shift generator. To safe some RAM I tried to put the shiftvalues into the PROGMEM but this actually does make some "strange" results and i dont know why. It seems that it returns the same value for a long time, than get back to regular expected results.

I hope you can help me and show me what I have done wrong.

const uint8_t shift[] PROGMEM =
{
    0x01, 0x01, 0x02,   0x01, 0x01, 0x03,   0x01, 0x07, 0x03,   0x01, 0x07, 0x06,   0x01, 0x07, 0x07,   0x02, 0x01, 0x01,
    0x02, 0x05, 0x05,   0x03, 0x01, 0x01,   0x03, 0x01, 0x05,   0x03, 0x05, 0x04,   0x03, 0x05, 0x05,   0x03, 0x05, 0x07,
    0x03, 0x07, 0x01,   0x04, 0x05, 0x03,   0x05, 0x01, 0x03,   0x05, 0x03, 0x06,   0x05, 0x03, 0x07,   0x05, 0x05, 0x02,
    0x05, 0x05, 0x03,   0x06, 0x03, 0x05,   0x06, 0x07, 0x01,   0x07, 0x03, 0x05,   0x07, 0x05, 0x03,   0x07, 0x07, 0x01
}; //needed as hex

static uint8_t y8 = 13;
static uint8_t cur_shift = 0, request_count = 0;

uint8_t rnd()
{
    //shift the shifting numbers
    request_count++;
    if(request_count == 255)
    {
        request_count = 0;
        cur_shift++;
        if (cur_shift == 24)
        {
            cur_shift = 0;
        }
    }
    //shift something around
    y8 ^= (y8 << pgm_read_byte(&(shift[cur_shift * 3])));
    y8 ^= (y8 >> pgm_read_byte(&(shift[cur_shift * 3 + 1])));
    return y8 ^= pgm_read_byte(&(shift[cur_shift * 3 + 2]));
}

Reference to the Numbers for the shift