Timer1 period register
PR1 should be set to the the number of timer counts you want in a period, minus 1.  To calculate that you need to know the instruction cycle rate.  You say the chip "runs at" 8 MHz, so I guess that means the instruction cycle rate, not the clock rate.  In that case there are 8000 instructions per ms, so PR1 should be set to 7999.  Keep the prescaler at 1.