php mail() function - Webserver

We are running few oscommerce stores on a dedicated linux server. These forms Contact Us, Pasword Recovery, New Customer Sign Up, Order Checkout on our website is having issues. An Example will be on Password Recovery form when a customer enters an email address and submit, it takes aprox one min to submit the form. Sometime it only takes a split second. So this problems happens on and off. This is creating problems specially when customer submit an order we are getting these messages when credit card info is send to the Processing Gateway: "Your script timed out while we were trying to post transaction results to it" So seems like a sendmail / DNS related issue. Need someone to help us troubleshoot this issue. Thanks