If the car has an automatic transmission, it could be the overdrive sensor. Try and disconnect the sensor plug at the the left lower rear of the engine (by the left front fender). Now go back on the highway for a road test and see if this clears the problem.
Make sure the windshield does not have any residue. Try cleaning with a film type cleanser. Being that new blades have been installed, try putting the left blade on the right to see if any improvement. Sometimes the arm needs to have slight twist put on the flat part of the arm.
Have your mechanic show you the damaged steel metal lines (have the car up on the lift) that he plans on replacing. Steel tubing going bad in less then 11 years is rare unless you live in an area where lots of salt is used when snow is on the roads. Labor in my area is $100 per hour.