That's an LA motor not a magnum, The idle motors wear out and start to fail. If you can still keep it running with your foot on the gas that's the problem. I had 90 that had 60k on the clock when i got it, Had over 200 when i sold it. No major issues other than the trans that had to be rebuilt.
Had the same issue with my 98, I drove it to work trying to keep the idle up by putting my foot on the gas even when braking, was not fun…. Changed the battery and all was good. Let us know how it turns out…..
