1995 Chevy Cheyenne

I have a chevy 4x4 ext cab 1995 chevy half ton pick-up. The problem with it is sometimes it wont start due to complete loss of power. I have tryed to change the ELECTRICAL ignition switch wire harness part because someone told me that could be it. Well now after $150 dollars later I went to disconnect the battery cable while the engine was running to see if it would stay running (as I thought/think that it is supposed to) and it did not stay running. Can you help me with any suggestions as to what it might be.
Starter cylinoid? Lose battery cables? I really cant afford a mechanic, as I am inclined to do my own work with some help. Please help. I have checked the charge on the battery, its just that one time it wouldnt even crank with a jump. When it does this there is no power at all, I'm stumped. The odometer reads 213,000 but I believe the engine has been redone at some time. It runs great when it starts.
Maybe its the alternator, will someone please just reply.