91 ranger 2.3L dual plug engine, loses spark intermittently in the exhaust side coil pack. spark comes and goes at will. pulls codes 88(dual plug input control failure) and 18(idm circuit failure). it goes from running great to crappy in an instant. plugs, plug wires, coil packs are all good. ignition module was tested twice, both times it tested ok. any help with this would be greatly appreciated. thanks