With the info you provided it looks like you input apply clutches are burnt and not holding as well as they should, or the input apply piston is in very bad condition. This clutch applies in reverse and 1st gear and is really about the only common link between the two. In manual 1st range 3rd clutch is also applied for engine breaking and extra holding power and this is likely overcoming the slipping input clutch, that and the line pressure is quite a bit higher in manual 1st to aid in pulling up or coasting down a steep grade. There arent many other posibilities and unfortunately means rebuild.
