94 Dodge spirit hard time maintaining speeds over 100kmh

have you ever had a tune up done ?

More than likely it's the oxygen sensor. I'm a little surprised it hasn't thrown a code, but that could be a different issue. I had the same issue with my '92 spirit, and that is what it was. No problem, except at highway speeds (100km/hr). Disconnect the oxygen sensor and try driving it without it for a bit. If that seems to work (it will kill your fuel economy, so don't do it for too long), then you know what your problem is. The oxygen sensor is located at the back of the engine bay, in the centre. You will have to remove the air filter housing to get at it.