Engine surges or "hiccups"

When I get my 2006 Sonata up to 55 miles per hour or higher and then left off a bit on the accelerator it surges or hiccups as if it can’t decide whether to speed up or slow down. If I tap the accelerator or let it get below 50 it usually stops. This problem can occur with or without cruise control.

You may have a TPS (Throttle Position Sensor) problem.

A broken or loose TPS can cause intermittent bursts of fuel from the injectors and an unstable idle because the PCM thinks the throttle is moving.

Also check for vacuum leaks.