hmm, i kinda had the same problem, now im in debt 3k.
i have an 89' auto and the speedo didnt work when i bought it, no big deal i thought, 2 days later it started to idle really low, 200-400rpm and it wouldnt shift right at start up, once i started driving it, it would get up to speed really slowly, but once it warmed up it was fine. well 3days after that it wouldnt shift into any gears at all, it just stalled out when i put it in gear. shitty, its been 3 months of it sitting in the trannie shop and mechanics and they just now found out it was the speedo, i have to get a new one from nissan.
but yeah, point of the story is get it checked out before it goes wrong in a major way. like mine.