It's strange the misfire has moved from one cylinder to another and a period where it ran perfectly in between. Because it's not misfiring on all cylinders it sort of eliminates a tuning issue or the coil/distributor. I know you have replaced the plugs - did you use OEM equivalents? And did you check the gaps before installing them? Sorry about the barrage of questions but it's a process of crossing off factors until the problem is fixed.