Timing

i got a 91 suburban, (350 TBI) I just changed the intake gasket on it, but now i am having problems with the timing. i marked the distributer before i pulled it out so that it would go back in the exact same spot. well it did go back in the same spot except now it wont run worth a shit. i adjusted the timing by the spects and it wouldn't even run, so i adjusted it by ear, and it ran great for about a week and then it started running crappy again. so i re-adjusted the timing and it ran great again for about a week. i think the computer is re-adjusting the timing. To do it right you are supposed to disconect the electronic spark advance, the book says it's a tan wire with a black stripe, but i can't find it. Can anybody help?

If it is in the same place as my 88 full size jimmt it should be on the passenger side firewall