Hi Miguel
  When you capped the hoses did you do anything, like insert a long
bolt say, that would impair the coolant flow?

Are there any kinks in your soft hoses? Water pump good?  Can you
disconnect at each end and blow out the lines with compressed air?
Try inserting a pice of clear tubing so you can see and analyze the
coolant flow.

Just a wild thought but might seasonal temperatures be a contributing
factor?

Does it overheat if you turn the front heater and fan on?

\How long have you had the SVX installed?

Is it possible that the vw rad isn't up to the task for the svx?

Just thinking out loud>>>

Don