Hot water heaters burner only kicks on when i turn the tempeture dial up by hand.?

burner quit turning on by itself. If i wash clothes & use up the hot water it wont kick on. but if i turn the temp. control knob up a little bit it kicks on then i turn it back down to hot it stays on until the next time i use the hot water up a little. I was wondering if i can maybe fix it by replacing the 5 dollar thermo coupler or is it the gas control valve or something else. PLEASE, can anyone help.........