Get a code-reader on to the OBD-II port and see if there any codes or pending codes showing. That may give you a clue. Do you have service history? Were the plugs and coils changed in an attempt to resolve a known misfire for example. If the intake has been changed, check that there is no coolant in any of the plug recesses. The gaskets can leak if the intake is badly torqued down (Dorman-type intakes tend to be more unreliable than the OE Ford or MGR intakes; do you have any indication of the make of intake was fitted?). If it has been raining recently, look for any sign of rainwater in plug recesses, particularly #8 is at the back, right hand side as you look at the motor from the front. That one gets the screen-washer jet/rainwater leakage sometimes.
As you probably realise, there are many possible reasons for a slight misfire, and it is very difficult to be more specific without some clues, or having the car in front of you.
Do you have the last 3 digits of the VIN, please? That may yield some information about the car's history which could help. I any event, it's a good idea to join the club as a full member, which will give you access to a wealth of valuable information, some of which is in Forum Sections for "full members only". Essential for a 260 owner.
Send a PM to Tim here to get a questionnaire (please excuse blatant advertising !!).
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