My 99 mazda protégé engine skips or hesitates for second at highway speeds only. Check engine light is on.
Has 120k on it, replaced spark plugs, replaced spark plug wires.. still doing same thing.
Could this be a coil/EGR/o2 sensor/ or fuel filter issue? Trying bring it down to most likely thing to cause this skip/hesitation at high speed. Thanks!

Read code(s), by attaching a code reader or scanner to the Data Link Connector under the dash. Many auto parts stores will do this for free. It may turn out to be a misfire situation, where one or more coil-on-plugs needs replacing.

Checking for codes will send you in the right direction.