2000 gtp will not engage in drive but can be manually shifted into D at the right speed. tranny slipping slightly in all gears except reverse. still have all gears available but wont shift in automatically. any suggestions on what to repair since i dont have enough cash to do a full rebuild at this time. this ismy first time doing auto tranny work but considering i cant afford to have it rebuilt i thought i would give it a shot. got to learn sometime.