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2008 Pontiac Montana SV6

HI! I have a montana sv6 2008 engine 3.9 lts 75000 km and
when I drive at 1800 rpm on lock up the engine sometime
skip 2-3 times before downshifting so had a spark plug cracked on cylinder #5 I chanded it and the problem is back
after ± 700 kms what is it?

is the spark plug cracked again?

Check engine light status, diagnostic codes? It sounds like an engine miss. But could be a transmission shifting problem too.