I had a simialr experience with my 2.6L. I pulled each plug one at a time until I narrowed down the culprit cylinder. I then removed the valve cover and found that the last Camshaft Bearing bracket had broken in two, allowing the camshaft to move upward and not opening and closing the valves properly. It was a simple fix, but you might check into that.

-Austin

1988 SWB Montero - 2.6L - 5-speed
Weber Carb
(Coming Soon) Rugged Ridge AVM Hubs