8000 instructions is not a whole lot and depending on the optimization level can easily be reached during procession of a timer isr. You may want to experiment with optimization levels (eg -Ot) and see if that makes a difference, but I would put a fair amount of money on the table to bet on your system being too slow.
Look up configTICK_RATE_HZ in Customization - FreeRTOS™ for details.