Electrical problem
1990 Other Isuzu Models 4 cyl Four Wheel Drive Manual

I have an 1990 Isuzu amigo 2.6 L when i go out to start it if starts right up then dies but if i keep the key turned to the start position it will run it seems that the fuel pump shuts off when i let go of the key but it dose not always do it manley when its cold i have repalced the relays and checked fues and wireing i cant fined anything but one thing that i have seen is i toke out the ingtion swich to check and see if its getting all the power and i seen that when it is not staying running there is a wire that has no power running though it but when it is running right there is power running though it and i tied to jump it from a place that has power off the ingtion and when its about to die i jump it it will stay running untell i take it off or or it startes working bye its self whith out me jumping it and i also seen that when it starts to work the way it should the engine idle rpms will go up and down and even wores when i give it gas o and one more thing i seen that when i jump that the O2 sensor and fuel & brake & battery lights stay on. on the dash can you please help me im