how old is the servo?
the split flaps on the FW190 aren't that big, the 3004 is a pretty stout servo and should hold the flaps up fine, I use them on all my 60 size warbirds and there isn't any sag at all when the reciever is turned off on any of the control surfaces, I wouldn't worry about the size of the servo, but the particular servo might be a little loose in the gears if its allowing the flaps to droop.