check engine light comes on at at around 45mph

My check engine light comes on at highway speeds and then goes out when I slow down. The truck runs just fine. The plugs, wires, PCV, were all just changed (problem existed before this). Any ideas? It doesn't come on any other times.
This is a 1995 with the 2.2 at manual tranny. How do you read the codes on this truck?

Thanks
Harley

P.S.
I just got this truck this week so I am going through it and fixing everything on it. Truck has 80,000 miles and and very nice ( I stole it for $1900)