There is definitely a buffering or audio processing issue with iOS 11.3; I have had issues on my iPhone 7+ since upgrading. Even when the iTunes song is downloaded onto the device, there is stuttering, skipping, and dropping out as if the "bandwidth" is poor. Note that this is using corded EarPods (multiple pairs). Signal strength, airplane mode, battery level (or low-power mode) all have no effect.
Regarding the AirPods, the issue above is mysteriously gone. BUT, every time the APs reconnect to the phone, they usually drop out per the OP within about a minute. Music is still playing and shows to be routed to the APs, but there is no sound. Switching audio playback destination (between iPhone/APs) usually "resets" it and there is no more dropping out after that. Turning off/on Bluetooth resolves it temporarily, as does re-pairing the APs.
I don't know if these issues are related, but they are both annoying.