ford escort zx2 engine stumble

I have a 98 ford escort zx2 that runs fine in town. When the engine is warmed up and on the interstate the engine will stumbe intermitintly.
Any ideas would be appreciated

When this goes bad what problems does it cause. its like my engine cuts out sometimes on interstate. just wondering if this could be the problem

I initially would suspect a vacuum leak.
Check vacuum hoses, and if you have the means to do so, do a compression check.
I had similar symptoms once and it was a result of a vacuum leak at the intake manifold... was pretty hard to detect. The only symptom was at highway speeds the engine would stutter a few seconds... was very intermittent.