Running 6 counters at the same time

I want to use 6 counters in NI PCIe-6612 at the same time to count pulses from 6 channeltrons, and I found that the counters miss some counts when I use TTL out of a function generator as a test source. For example, it shows ~40 counts in one second when I apply 60 Hz TTL pulse. The fluctuation is also quite large (~30%). When I use only one counter at a time, it reads more accurately, but still ~10% less counts than what it should be.. Is it possible that the computer cannot take the load since it's quite old (about 8 years, Pentium 4 with 1GB RAM).