Bobcares is good and has good rates. Arress India - http://www.24x7hostingsupport.com/ is another good one. With both bobcares and Arress, you can pay by the ticket / customers, or if you can justify it hire dedicated techs that answer only your issues. We've worked with both of those companies fairly extensively and certainly have more good than bad to say.

Will they give you headaches too sometimes? If you use them long enough ofcourse, but that's just tech support. The headaches are far fewer than managing an in-house team 24x7.