Weber gas grill wont heat past 300 degrees

i have a weber silver C gas grill. new tank of gas, and it wontheat past 300 degrees. i thought it was a bad tank not letting enough gas through so i got a new tank. same problem. i followed the directions for resetting the saftey valve and same problem. i have checked the burners they are nice and clean. no kinks in the hose. what could be the problem? any ideas? regulator? burner control? there isnt much to the grill it doesnt seem. i really miss grilling!
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