check the engine lite flickering then on.

i have a chevy s-10 crew 4x4 with 90,000 miles. replaced the antifreez, plugs, wires, rotor, and cap. while pulling an empty 12ft flat bed trailer the check engin light comes on. it first flicks about 3 or 4 times then stays on.
today i was going up a short small hill , gave the truck a little gas and the check engine light started flashing. I pulled over and turned the engine off. started the otor back up and the light did not come back on. later down the road started up another small incline, the light flashed 3 or 4 times and stayed on. what can this be?

A flashing check engine light points to a cylinder miss or miss fire.
Check for cyliner miss or missfire.
What kind of plugs did you use?
What engine?

If its the '91, the ECM does not have that capability to flash for a missfire. Read the codes, see what you get. If something is there(most likely) post back here and we'll see how we can help you.