Event counter not proper count more than 1800Hz frequncy

hello , everyone right now i use AT89C51 MCU with 11.0592MHZ and my task is i have count pulses of maximum 2.5KHz , right now i'm able to count the pulses of 1600MHz and it give right count value but when i apply 1700Mhz pulses it not count pulses proper.
my logic to count pulses is
CounterInit()
{ TMOD |= 0X50; // configure timer-1 in mode-2 TL1 = 0x00; TH1 = 0x00; TR1= 1;
} // This counter value loaded every 10ms unsigned char x, y; x = TL1; y = TH1; TR1 = 0; TL1 = 0x00; TH1 = 0x00; TR1 = 1; count +=(y*256)+x; lcd_com(0xa2); lcd_dispint(count);

i use Timer1 as a event counter. now what i make mistake? is any idea how i get 2.5Khz pulses count proper please tell me and if possible and you have any code in C than please send me tejanihiren1991@gmail.com