While driving at approx 55mph, my RPM's dropped from 2500 to 1500 and the truck started shaking. took my foot off the gas and re-accelerated went back up to 2500 RPM's and ran normally for a bit, then happened again. it's happened several times over the past week. shifts normally in and out of overdrive and seems to shift normally at other speeds, does seem more sluggish than normal, no strange noises, check engine light flashes at 55mph...what does this mean?!