Codes P0303, P0304, P0305 and P0306

Originally I had a P0303 code on my vehicle. I decided to be proactive and take the opportunity to replace all plugs and coils with brand new ones that were exactly the same types - both Denso. After replacing, the vehicle ran fine for a few days but then out of the blue on driving, the vehicle started to cough and sputter for 10 seconds or so then ran fine. Checked the codes at home and had 4 codes show up - P0302, P0304, P0305 and P0306. I checked for loose wires, hoses disconnected and found none. I did find a grounding point on the frame that I thought was the culprit since it was badly corroded so I cleaned it off. I checked the others and well and cleaned them. It ran smooth as silk for another three days and again out of the blue it started to cough and sputter for 10 seconds or so. Got home and checked the codes - P0303, P0304, P0305, P0306. It almost seem to be an electrical short somewhere but I'm not sure. Any ideas?