Ford Escort - Why does "check engine" light turn on?

[Dmitry Samsonov]

Recently I bought Ford Escort 1991 with 1.9 l fuel injected engine. It runs fine, 
but if I drive on the highway at about 60-70 mph "check engine" light comes on.
If I press the accelerator for a few seconds and then release it (and return 
to cruise control) the light goes off for a few minutes and then comes on again. 
It is never on if I drive in town. I changed the spark plugs recently 
(it did not help). The car manual says that this light is controlled by ECU
and if it on for a minute or two it can be ignored and if it is always on,
the car should be check with a mechanic. It stays on almost all the time 
I am on the highway. The car has high (117,000) mileage.
Does anybody know what might cause the ECU to turn "check engine" light on and 
is it an early warning of an engine problem or just one of the sensors got bad? 
Thanks.

Dmitry