Sounds like might be a coil pack. That vibration you experience is probably the roughness of running on only 7 cylinders.

Bring it into the shop to have the computer tell ya which cylinder it is. If it's cylinder #4, have them check the coolant plumbing near-by and tighten any loos fittings.

Please let me know if this was it. I just fixed mine for the same reason. BTW, cylinder #4 is up and under the damn cowl.