Possible electrical + transmission problem(s)

[StockerThanStock]

I've got issues with a '91 240sx and I have no clue really what's wrong.

First off, the automatic transmission started acting up after a rebuild after only 1500 miles (and it's not covered under the warranty!). Every once in a while when I start driving, it's ::like:: it goes into safe mode and stays in 3rd gear going forward. There's no engine codes being thrown up though which sort of confuses me...

Next, if I have the radio playing and turn the headlights on, the light on the radio goes dim like the headlights just drained half the power going to it. Would this be the alternator going bad?

Also, if the alternator is going bad and I'm running off a weak battery, could this cause the transmission to act up and go into a safe mode? Is there anything else I should look into before I pry this alternator out to get it patched up?

[Florida240sx]

Sounds like alternator/ground issue.For transmission most people have it hang up in first gear... It's the revolutionary sensor. AS long as your speedo works then you can just unplug the sensor. It's in your engine bay behind the battery. Been 8months no problems since. Big thread on it...