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can overfilling affect the operation of the box
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Yes - overfilling is bad for auto trans. If the fluid level is too high, some of the spinning parts will hit the fluid and the fluid will foam. Then you won't get any hydraulic action - or greatly reduced.
It sounds like your clutch packs are not fully locking up until you get to higher RPM - which is also higher pressure on the clutch packs. Usually, once an auto starts to slip, it is only a short time till all the clutches are burnt past working at all. It's always worth a try to change the fluid and filter and hope for the best.
Good Luck
Tranny Source Rex.