I have a Mitsubishi Galant 2005 which was running fine only engine oil was leaking in spark plugs compartment. during the routine maintenance i asked my mechanic to fix this & also the timing belt because run was 160k.

my mechanic changed the belt & other thing now engine runs quite & smooth but the problem is when i exceed the speed on highway around 100 - 120 one coil shuts down.

i changed that coil but it is still the same can anyone help me in this regard.