2003 S-10 "check engine" light

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guys,
my 2003 s-10 extreme 4.6 v-6 with 65k miles.
when i get on the highway and am going 65-75mph my "check engine soon" code will flash on and off fast for about 20 flashes, then stop for about 30 seconds, the start the flashing again.
it will do this for 10 minutes or so, then stop.....
do you got any ideas what is going on ?
cant use my scanner because it wont stay on...
the truck is running perfect.