the difference in driving in 3rd -vs-OD,,,other than not able to go to OD,,is the line pressure /control pressure is higher....damaged or worn foreward clutch will engage better because of higher pressure.......you also could try disconnecting modulator,,this will allow trans to go to full line pressure,,,see if it will work somewhat better.....don't have a flow chart in front of me ,,,but this is one scenario.......(did i spell that right?).....probably going to need some work though.....