Strange Packet Loss with simple queues!

Well, I have a local net, all PC’s connected to simple SWITCH 24 port (just like HUB)
also I have RB450G with OSv4 where 1 port connected to ISP and second port to SWITCH

PC1__
PC2 – [SWITCH] - [RB450G] - ISP
PC3 _/

All I need is to limit PC1 and PC2 and leave PC3 unlimited, and I did it by using Simple Queue
where I put

[admin@MikroTik] /queue simple> pr
Flags: X - disabled, I - invalid, D - dynamic 
 0    name="PC1" target-addresses=192.168.1.1/32 dst-address=0.0.0.0/0 interface=ether2 parent=none direction=both priority=8 
      queue=default-small/default-small limit-at=0/0 max-limit=512k/512k burst-limit=0/0 burst-threshold=0/0 burst-time=0s/0s total-queue=default-small 
 1    name="PC2" target-addresses=192.168.1.2/32 dst-address=0.0.0.0/0 interface=ether2 parent=none direction=both priority=8 
      queue=default-small/default-small limit-at=0/0 max-limit=512k/512k burst-limit=0/0 burst-threshold=0/0 burst-time=0s/0s total-queue=default-small

all went good until PC1 start to download big files, in this case I got about 14% packet loss! but it was still limited as needed.
The problem that packet loss affect all PC’s I have, even PC3 start to get packet loss, but it’s not even in queues!!!
When PC1 and PC2 dounload less then LIMIT, there is no Packet loss.

I have no added nothing else in any Firewall etc. just this.

Please let me know if I make something wrong over here.
Thanks