1994 Grand Marquis LS Check Engine in Higher Speed

I recently had my car fixed for a few normal wear and tear problems (she's nearing 140,000) but something that wasn't fixed was a check engine light. It only pops up when the car speed goes a little above 70 mph, or when accelerating on the highway. It stumped my mechanic, won't come on when he drives it, only when I do. Any reason why it's coming on? It's rather annoying, and only comes on I guess in overdrive, and at higher rpms.