I have been getting backfire in high rpms, right before the engine shifts. This started around the time I started using midgrade gas instead of premium, because of the price. Is this just a coincidence, or could it be because of the gas? Last summer I changed my plugs and wires, but not the coil. I'm not sure if the problem is as simple as the gas or if it's ignition related. Thanks.

 

switch back to premium for about 2 tanks and see if it remedies the problem...low octane + summer could mean detonation

 

Yep, sounds like it. Try some premium first. If there's no detonation and you want to continue running lower octane gas, I'd recommend pulling out some timing from the programming because it's pretty aggressive for mid grade.