Roof Ventilation and Insulation - Midwest - Gable and Ridge vents only, no soffit vents
Here is your problem Gable Vents and Ridge Vents do not go together period.

Your Gable vents are acting as Inlet Air Ducts for your Ridge Vents.

So your air flow comes in through each gable and flows immediately to the closest air outlet the ridge vents NEAREST the gables. 

So the Air flow in the center of the Attic is basically NON-EXISTENT . And the Actual cooling from Gable to ridge vent is very limited. 

Either one or the other but not both.

So here are some options:

Option 1: Close your ridge vents.

Option 2: Close your Gables and Install Soffits (I know you don't want to hear that - keep reading though) - these would need to be properly installed - a soffit guide is best - so your insulation and critters do not block them. 

Option 2 will allow you to get rid of your Electric Fan as well, saving you money on the electricity of the fan. As for how much difference Ridge Vents vs. Gable Vents I can not tell you if there is a savings or not a savings, getting that info is like getting an act of congress for something that matters.

With Option 2 you have direct line air flow from outside upwards to the roof all working by convection - $aving you $$ . 

Now if you do this a vent here and there might be expensive and time consuming aside from calculating how many and what size .. Instead of doing that - they make Continuous Aluminum soffit that is pre-drilled/cut with holes. You basically lay it in like aluminum siding and it is very easy to install. When I was living in FL my house there had it. This type of soffit is inexpensive, it works great and as I said easy to install. I had to replace a couple 3 foot x 6" sheets; if my memory serves me right - I think it had cost me somewhere around $8.00 - I think it was so much cheaper than what I thought I had bought a couple extra with out skipping a beat. Probably a bit more expensive than that now - I went to a local metal soffit fabricator shop and purchased it.