If it's still the original carb and not rebuilt, it is possible that the 2 computer controlled components are dying. The computer controls the mixture, and there is also a throttle position sensor in it. Either of these can cause problems if not operating correctly.
Does the car have a functioning check engine lamp?
My last Bonneville wagon ran worse and worse...discovered the previous owner had removed the bulb from the CEL, car had been throwing codes for BOTH the TPS and mixture control solenoid. Reman carb made it run like new again....but it had already started killing the cat converter due to all the fuel it was dumping into the exhaust.
Recently acquired 81 wagon had a non-computer carb on it...would not idle, hard to start, took forever to warm up as the choke was not connected. I replaced it with the correct CCC carb, runs great, idles like designed, The computer carbs get a bad reputation (like many things people don't take the time to learn about), but when correctly operating, they work very well, and are calibrated for the engine. If you make a lot of changes like headers, intake, gearing, then changing to a different setup will be advantageous, but for a stocker, there's not really anything wrong with the OE setup when working properly.
