2002 jeep Cherokee check engine light flashing sometimes

I bought a 2002 jeep grand Cherokee Laredo 4.0L
recently rebuilt. There was no problems with it
when I bought it. But after about a month or so
there is a check engine light on. The check engine
light starts flashing randomly. It chugs to begin
with but when light is flashing chugs very hard and
loses alot of power. And if you just coast after
awhile itll to to a steady light. I also found out that
if the light is flashing and then you turn the jeep off
then back on, the problem is fixed besides the
check engine light still on. Does it randomly and we
took it to a shop and they just  replaced coil pack
and plugs. Nothing fixed.