I was having this exact same problem just a couple of weeks ago on my 1997 Disco V8.
It was being caused simply by a lose wire on the coil so I would suggest checking all the connections on all the wires going to the coil. Even if they look good, just take them off, a quick clean and reconnect.
I've not had the problem again since doing this and it was kind of a relief that it was something quick, cheap and easy to fix unlike all the other things that seem to go wrong on my Disco.
2012 Discovery 4 SDV6 HSE
2003 Discovery 2 TD5
2003 Defender Xtreme
1997 Discovery V8i
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