When a car is surging while driving down the road or highway it's usually because the engine is not getting the proper amount of fuel required to keep the engine running at the speed you're driving.
The engine surging because of the fuel not being enough is because the engine is running too lean.
You could have a bad fuel injector, clogged fuel injector, clogged fuel filter failing fuel pump etc.
Besides clogged or restricted or bad fuel injectors a vacuum leak through hoses, intake manifold gasket etc can also cause an engine or car to surge while driving.
You should take the vehicle to a mechanic or if you're able to work on it yourself check the problem out and get it fixed before it leads you to be stranded.
You can get a computer diagnostic OBD tester and test the car yourself and it will throw you some codes that you can look up online to see what is going on.