Why does my check engine light go on intermittently?

Hey truck guys try this one my 2000 Dakota
has just had major electrical work done
because it would crank but not fire after my
mechanic replaced distributor,plugs ,wires
and everything else necessary she purrs like a
kitten Now the issue after about 250 miles the
check engine light goes on I take it back to
him and the computer readout says misfire in
cylinders 1. 3. and 5 which is nuts because
she runs beautifully  This issue has reoccurred
numerous times and each time he clears the
codes I am good for a day or two or a week
then the light goes back on Same misfire code  
Any suggestions? Thanks