need transmission answers

have a 90 plymouth voyager that doesn't want to shift out of first a lot (now), and when I turn it off and back on, it fixes it most of the time. I've been asking all over town and the internet and have gotten a range of answers.
One person has told me that it's a tranny computer problem and I can simply reset it where it is hooked into the battery. First question: is this feasible, because if the computer is acting up, then, won't it still be when I reconnect it? My problem is solved almost each time I turn the van off and back on again, which has been a temporary solution to my problem at this point.
One person says I need to get the diagnostics first, so my second questions is what does CVI stand for and how can I use these if I don't trust the mechanic?
Also, if this tranny computer resets when I turn it off, then, how will a diagnostic help?
One person says, it could be my speed sensor, so will a diagnostic tell me that this sensor doesn't work?

Since I've posted this, I've got the fault codes from the main computer saying, 12, 15, 51. I haven't disconnected the battery or any power from the battery at all, since it was installed in December. Could this be a grounding problem with my transmission? :eek7: