"Check Engine" when going up long hills

Hi, I deliver pizza in my 1992 Mercury Tracer, mileage 190,000 (and going strong!). My concern is this: I deliver in a rather hilly area. When going up long hills, like over 30 seconds to the top, I get a check engine light by the top, and if I drive for about 10-15 more minutes, it goes away. I figured it's probably an oil issue, so I replaced the oil and filter, and even dropped the pan and flushed the whole system out, but to no avail. Anyone else get anything like this?