Yea a08 is def not the right one. I had a brand new a08 and it stumbled terribly. There's a forum member who bought one from an eBay seller for 140 shipped and it had a09. Might be worth a try.
These systems are very picky and even wen they work properly they do have that power loss when it enters lean burn. Try unplugging the o2 and see if the stumble goes away.
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Burn lean and prosper \\ //
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