Hi,
Think all that extra weight it has to carry or more power/torque may have something to do with it..?? j/k
I don't know why you say it's a solenoid? The only Solenoid in the unit is the Shiftlock Solenoid and that lives in the Center Console. It's purpose is to prevent you from shifting into reverse(or any gear from Park) unless you have your foot applied on the Brake.
I suspect that either it's low on Fluid, have you checked this, or for possible leaks? Or that it's been beat to death, in which case, only replacement is an option because Porsche doens't make internal parts available for the unit.
In fact, one of the local Dealerships here said they don't even have a mechanic familiar with the unit because they've never had one fail, the Tiptronics are pretty bulletproof.
This leads me to believe that if you are having problems, abuse is likely high on the list as to the cause, and replacement probably your only option. Hope this helps...
Happy Motoring!... Jim'99
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