[12Ounce]

If it also is sometimes hard to start up, I'm guessing the fuel pump relay. See your glove box manual for location inside the battery fuse box. Replace only with a new one if you possibly can...in other words, avoid swapping relays about.

My other guess would be the IAC valve. It's mounted atop the intake throttle body. Easy to get to. Check the connector. Some folks have had, at least short term, luck cleaning the inside of this valve with carb cleaner and WD40.

My other-other guess would be faulty cable connections to the battery or ground cable to engine block ...or just a faulty battery. This is the loooong shot guess,.... at very slow rpm your system is dependent on battery for electrical support....at higher rpm the alternator can "carry" the system and a failing battery or bad connections.