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.

SHUIAB
March 2011