how do i fix my car ?

i have a merc c200 1998 modle auto / i have already fixed my gearbox in april last year . i changed everything . but i sill have a problem of witch the car go's on a safty mode or as some call it limp mode . when i put the diagnostic machine on and clear the fault the car runs very well for a week or two . i dont know what else to do . i have changed about 3 or 4 valve bodys . which cost about r3000.00 each. done the loom. done the drums inside the box and the clutchs. please help me do'nt have money now to spend on this car.