I have a Zafira with the X18XE engine and those symptoms are awfully familiar.
Been through two thermostats so far, they're notoriously prone to failure. QH do a repair kit containing just the 'stat, rubber-ring gasket[1] and a cardboard tube to remove / insert the thing with. GM will only sell you the entire 'stat, housing and temp sensor assembly for rather more pennies.
[1] If it's a similar housing assembly to mine, that's worse than useless. Use some instant gasket instead.
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