Fuel Injection problems of the 1999 Chevrolet Astro

When vehicle is in a starting position it shakes and loses power.

Engine misfires and runs poorly setting off the "check engine soon" light. The first time we took it to the dealer I was told this is very common in Astro vans and I should not use chevron gas. I thought this was strange advice because I never used than brand of gas. I had a record off all the brands of gas I used but the dealer was not interested and I was only told to switch brands. The second time I brought the van in for the same problem I was told to switch brands again. Now there are no brands left to try. Each time the dealer added a clearner to the gas tank and idled the car for 45 minutes to "clean" the sticking injectors. The van also gets very bad gas mileage. I wrote a letter to gm and I was told that it was a technical problem that the dealer must solve.