cel flashing 89 f250 460

[desertdeer]

i have an 89 f250 4x4 with a 5 speed. when i got it it wouldnt warm up and got horrible mileage. i replaced the thermostat, then the coolant temp sensor. warmed up quicker and got better mileage. but since then i have noticed the check engine will flash when cruising, and lightly accelerating. it doesnt do it when its fully warmed up or dead cold. just when its in between. i havent been able to get any permanent codes to read.

any ideas????