Nissan Repair: 89 240sx weird electrical problem with bogs, 89 240sx, driveability problem

Question
Ok here is the deal I bought a salvaged 240sx and they say  it was pieced together from a junk yard no frame damage what so ever,  
so after i drove this thing for about 3 weeks i noticed the speedo did not work the it started working on and off now 3 months later it goes crazy from 10mph the it will shoot past 100mph and sometimes it drops all the way to the right when this happens the engine check light will come on an the weird thing is that if I am on 3rd or 4th when I push this past 4k rpm it bogs out light a rev limiter and it will come back after it drops past 3k rpm at this point check light is on and needle is stuck and i have to wait and cruise till it goes away. I  read online and some dude said to hit the dash  
so I tried it and it works it gets the needle moving and the  light goes off but i mean I have to smack the hell out it other symptoms is when I start the car It almost doesn't want to crank but always does lights are dim until l i rev and after warm up sometimes it stays at 2k rpm idling until I smack the dash well  below are the parts I have checked and replaced
Oh almost forgot my fuel empty light does not come on but gage works
battery,alternator,gage cluster ( same effect)
I have checked connections to ecu in the back of the dash under dash fuses relays

please help i tried to tell u all I know

Answer
Hi Jay -

It definitely appears you have muiltiple issues with your vehicle. Because it was piece-mealed together, it is hard to tell what is going on, many sensors could be missing, wires crossed, connections lost, etc. Your immediate issue seems like a weak battery, so I would change that before going further. IOf you have no panel lights, check fuses and bulbs. If that is all OK, then probably there is a non connection somewhere.

As for your driveability problem, You will probably have to research the engine light and place it on diagnostic to find out what is making that light come on. Could be as simple as lubricating your speedo cable, as there is a speed sensor and if the speedometer is stuck at 100MPH, the computer will sense that and think you are actually traveling that fast, and will adjust the engine to that speed. Because you are actually not going that speed, it probably is overfeeding gas, and that is why it bogs.

If it is not too late, I would take it back if they offered you some kind of warranty.

Hope this helps
calvin