intermittent sporadic RMPs

Hi. I'm driving a 5 speed v6 Mustang. Twice in the last week the car has got into this state where the RMPs become very sporadic (so the car is randomly pulling and then slowing several times a second). At one point I was at the light idling and suddenly the RPMs wanted to rev up to 3 or 4 thousand RPMs, followed by wanting to stall. This lasts for anywhere from 3 to 10 seconds and does it a few times before returning to regularity again. While it is in this state, the check engine light starts flickering rapidly. Once the car settles down the check engine light is off again. I've replaced my spark plug wires and spark plugs relatively recently. Most of the time the car is fine - this problem is intermittent. Any ideas?

Problem solved. The issue was that the Tuning chip (little flash rom device plugged into the ECU) had a bad connection to the ECU. Each time it would re-establish connection it would reset the ECU which would cut the fuel pump for a second. Once the ECU finished resetting it would dump fuel in to bring the RMPs back up. My ECU connection was failing like every few seconds sometimes causing my car to buck violently as the fuel is dumped in, then cut, then dumped in again, etc. The solution was to just remove the Tuning chip. My car runs perfectly well without it - I'm assuming using default ECU timings/settings instead of the ones on the Tuning chip.