It's been rebuilt twice and has failed again? I would think that's a problem with the rebuilder. You need to find a better shop or buy a rebuilt unit - from Sun Valley or Adsitco - they're both around $1200 for four speeds. I'm not sure what a conversion to a five speed would entail but you wouldn't want one anyway. The four speeds are more reliable and less expensive to repair - for example check out the prices on Adsitco.com / online catalog / drivetrain / transmissions-rebuilt.