Victa Mower wont run.

Hi All,
Is any one able to offer constructive advice? We have a Victa Corvette 400 mower. The problem is that it loses revs and just before it cuts out it picks up the revs again. It is continous in about a 5 second cycle. We have cleaned the carb and changed the spark plug, renewed the fuel and changed the air filter. Is there any thing else I can do before submiting defeat and paying to have it repaired?
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