for the idle...i can tell you on my honda it did the same thing, and i thought it was a vacuum leak, MAP sensor, Idle air control valve...etc. went over/replaced it all and the damn thing still surged from 1-2k. we ended up blocking off the ports for the IACV in the throttle body and now it idles beautifully...you just have to hold the gas yourself for a fast idle when cold. i just dont know if you have TBI or MPFI, so i dunno if that's an option for you...
as for the shitty running recently... maybe the fuel pump?
the last v8 that i had did that (but it was carbureted), so i thought the carb needed rebuilt/tuned, so after all of that and playing w/vacuum hoses it still would cut off, need to "clear out" and wouldnt idle at all. ended up needing a valve job (but that was on a '74 w/ original motor...back when they used the same valves for the "new" unleaded gas motors that they did in the leaded ones...it was explained to me that they F up regularly on those valves..........but obviously that's a very different situation....)
you said it had 200k on it so i'm just throwing out ideas
GL man