95 c280 will not start
 

this c280 (1995) ran fine at first and then every once and awhile it just would not start but after a few tries it would and run almost perfect minus light rough ideal. it soon wasn't starting once a week and now it wont at all. here is what i was going to replace:
fuel filter
spark plug
overvolt relay
my question is the the overvoly relay and the fuel pump relay the same thing?
does anyone know the new part number of the new overvolt relay?
any other sugestions would be great
thanks
Chris