1991 ford ranger check engine light

1991 ford ranger2.3L dist-less ign., check engine light on intermetant, shows code 18, when the light is on it stumbles and misses and the tach reads about 1/2 of the RPM, when the the light is off the tach reads right and the engine runs real good. My manual does not show where the tach signal comes from, is it from the CPS (crank position senser) or from the Ignition modual?? If someone could help I would sure appreciate it.
I have had the coils, CPS, computer changed. I also checked and cleaned all grounds. Also is there a certain way to reset the computer??

1991 ford ranger2.3L dist-less ign., check engine light on intermetant, shows code 18, when the light is on it stumbles and misses and the tach reads about 1/2 of the RPM, when the the light is off the tach reads right and the engine runs real good. My manual does not show where the tach signal comes from, is it from the CPS (crank position senser) or from the Ignition modual?? If someone could help I would sure appreciate it.
I have had the coils, CPS, computer changed. I also checked and cleaned all grounds. Also is there a certain way to reset the computer??
I replaced the ignition modual and that fixed my problem.