The RB5009 has issues if the switch traffic ingress port runs at higher rate than the egress port, in your case towards PC2 with 100MBit.
There are several topics discussing this, the biggest is this one, discussing the same issue when mixing 2.5GB and 1GB.
http://forum.mikrotik.com/t/rb5009-support/152371/86
The release notes for 7.2 claim to fix this:
*) switch - improved packet forwarding with enabled “cpu-flow-control” setting between different rate interfaces for 88E6393X (RB5009) and 88E6191X (CCR2004-16G-2S+) switch chips
This brought some improvements with this issue, but it is not fixed. There is obviously something wrong with Ethernet flow control between the CPU and the switch chip, which is required exactly for such situations:
For now, there is nothing else to du but wait for MikroTik to acknowledge the issue (MT support claims it does not now about) and than hopefully release a fix with future ROS releases.