1992 Infiniti Q45 car starts but dies and fluxuates in RPMs

Engine Performance problem
1992 Infiniti Q45 V8 Two Wheel Drive Automatic 250k miles

I am trying to figure out why my infinity will start every time but will not hold an idle RPM and when I rev the engine it harmonizes at about 3 rpms and runs terrible. If I press the gas harder it will break through and reach higher rpms. If I hold at 5 rpms it will fluxate from about 4.5-5 and just when I think the engine is holding strong during the rev I slowely bring it back down to idle rpms. It may or may not hold for a few seconds and then the rpms drop and it just dies. The car will drive but I have to keep putting it in nuetral and reving the engine to keep it running other wise it will die even when in motion. Please help. I will donate but cant spend alot as I know Im looking at possibly $700 dollars just for parts