the game will run great for about 10 minutes or so and it varies from time to time, but then it will just slow way down and there's a warning that says like cl_flushentity or something like that. and it have to wait a bit and restart the game again. any idea of what's going on?

Sounds like a connection issue.

What are your rates and your connection speed?

cl_cmdrate
cl_update
rate
cable/dsl/satellite/dial-up