Engine is surging sometimes

Without doing other tests, it sounds typical of a plugged pickup screen inside the gas tank. Fuel pressure will drop low enough to cause running problems when the largest volume of fuel is pumped, ... Which is at lower speeds and especially when coasting. At higher speeds, fuel pump volume goes down so it's easier to get enough fuel through the screen to keep the pressure up.

To test my theory, disconnect the vacuum hose from the fuel pressure regulator and plug it, then drive it like that at lower speeds. You might see black smoke from the tail pipe, but if the running problem doesn't show up, suspect a fuel volume problem.