John,
most of the time it is the rudder itself.
Rudders on a Hobie 16 must be extremely stiff to prevent a spin out.
That can only be solved by using slim profile and stiff rudders. Carbon Fibre is the best solution nowadays.
If you have Lexan rudders, okay, you will always have that problem.
If you have the old black EPO rudders, the problem will be less but still existing. The factories do not such a good job on rudders, even with the white rudders you have a too thick profile that let the rudders stall, if you sheet in too tight and move the rudders extreme.
Solutions are there on the market.

Another practical one: Do not move the rudders so quickly, but try and steer softly that may also help.
Raking the mast needs to be "counter" raked with the rudders, you have to rake them forward, so that the weather helm is gone. But rake only is not the problem for stalling.

Hope this helps a bit. Happy to tell you more.

Fritz