Fault code P0304 but engine runs ok.

[TomUK]

Engine misfiring many months ago. Warning light came on. Changed plugs and problem solved. Used the vehicle for months without any issues, but the warning light stayed on. Didn't have means to cancel it.
MOT time came along and placed a discreet strip of black tape over the light.
Car Passed the MOT without incident, emissions ok.
ODB II Code Reader received at Christmas (useful present!). Three codes detected.P0304 (misfire all cylinders), P0711 (transmission fluid temp.), P1B00 (Undefined?).
Regarding the misfire code, the engine runs normally under acceleration, normal cruising and tick over is normal too. I suppose transmission fluid temp. could be a faulty sensor.
I erase the fault codes and switch MIL (warning light) to OFF and car runs for a couple of days without light re-appearing. Then out of the blue, it lights up again.
I am thinking about changing the coil pack, but with the car running fine, I wonder whether this is necessary. Any one else had similar trouble?

[TomUK]

Should have said changed plugs as one was wet & oily, which seems to have been causing the misfire.