1993 Chevy Blazer Dash has left the building.

Thanks for this great forum to come to.
My honey and I have always had older vehicles, easy for us both to change water pumps, etc. My father passed away and left us this nice Blazer-Tahoe, electronic everything. A couple weeks ago, on the freeway, the dash blacked out. The only thing left has been the mph. Now we have both tried to pinpoint this down: When we start it in the a.M, and you go about 5 miles exact, the odometer starts scanning thru all numbers it could ever show, and this goes on for about 10 min. No matter how fast-slow you are going. Then it says ERROR and no number reading, for about 10 min. And then it starts scanning again. My wife checked the fuses and I was wondering about the dash harnesses. We have both played with combinations of heater-radio-wipers-pwr windows to no avail. No change.
Also, happens on the freeway, on pavement, parking lots. We are not on any bumpy gravel roads.

Have gone to eight mechanics and they all say oh around $2000.00 after looking at it for only about 5-10 min.

Now once you get somewhere and turn it off, and (important) let it sit for at least 8 minutes, it "reboots", because when you start it up again, it is fine, until the next 5 miles has passed.

Please help us here, we reallllllly appreciate any help and pointing us to how to fix this. Thank you.