1996 Chevy Blazer high idle after mwchanic fixed it n much

My truck stalled at 70 an hour and would start and went a mile then stalled then started again went a little n died then I took it to a mechanic he had it ten days said he cant fix until I was taking itout then all of sudden its running n a 900 bill now he said the 5th cylinder is not working until I drive it a while but I drove it 500 miles n no change now its ideling high n running rugh like it wants to knock out only wen stopped and my check engine light is constantly flashing fast n never stops flashing I changed all plugs n wires n cap n its wasting gas bad I want to fix it can someone help