dns cache transparent webproxy big problem

excuse my english…
i need help to discover the problem in the configuration of dns cache and transparent webproxy

I am occupying single these 2 rules and adsl pppoe client

masquerade
/ip firewall src-nat add out-interface=pppoe-out1 action=masquerade \ comment=“Masquerades everything leaving the external interface”

webproxy transparente
/ip firewall dst-nat add dst-address=!192.168.2.0/24:80 protocol=tcp action=redirect to-dst-port=3128

everything in the beginning works well… after of hours dns cache lets work and does not work pop3

pc config: amd athlon 600 - 256bm ram - HD 20gb


ip address: 192.168.2.1
netmask:24
ip network: 192.168.2.0
netmask:24
ip dhcp: 192.168.2.1
ip gateway: 192.168.2.254
ip pool: 192.168.2.5-192.168.0.253

thanks!

please any help?