1995 Pontiac firebird formula 5.7

So im having a terrible time trying to get my car
running again. So it will start and run great on
jumper cables but when i take them off it dies after
about five seconds no matter how charged the
battery is. i bought both the battery and the
alternator within 2 months ago and thought well
maybe one is bad so i took them both to autozone
and both tested good. when i go to start it without
jumper cables it acts like the battery is dead and
just clicks like its trying to engage but doesn’t have
enough power. i took the gauge cluster out and
tried to wire up a newer one from a camaro and i
got it to turn on but nothing actually...... well worked
so i said whatever and put everything back together
and it seems like ever since then its had this
problem and i took the cluster out thinking maybe?
but no. didnt do anything still has the same
problem. Any answers or ideas.....?.....??