DUDE! dont put washers between the pressure plate and the flywheel! This will even further reduce the clamping force against the clutch plate, making the problem worse.
How much freeboard does the pedal have? I just bought a 310 sunny coupe who's clutch slips under heavy use. BUT< there is no freeboard in the pedal, so I belive that backing off the clutch adjustment will fix it. How far from the floor does it engage? If it's right at the top, then thats your problem. If not, then get your pressure plate tested by a clutch shop.