Server :: Squid Internet Access Getting Slower?

I've a SQUID proxy server installed in SUSE 9.0 ES server. I've created cache dirs on seperate partitions for better caching. Its working fine. But since last 15-20 days, i've experienced very slow net access to clients. I've gone through the /var/log/messeges file, it generates a two line error messeges

client read request fd602 invalid request
parse http request: unsupported method;HET

This messege increases as the number of clients increates (for internet access). The apperance of error messege lowering down as soon as the number of clients reduces.

As the count of clients increases error messeges increases, internet access getting slower and slower.