Yes, choose a spur and stick to changing pinions as the tyres wear - most do that.
If you are running 35s and everyone else is running 32s, that's the more likely explanation for the back losing grip in the turns.
I don't run Oval, but having the front lower than the rear will promote oversteer. Is that the usual setup the fast guys use at your club? When in doubt, put the common settings everyone else uses on your car and then try the differences you find between the top drivers. Start from there to try things, only ever changing one thing at a time.
Compare everything you do to the lap times you run. You are interested in your fastest lap time, but more interested in the difference between your fastest lap time and your average lap time over the race. Look at the top drivers 'difference'.
In 12th, the top drivers have a 'difference' of about 0.2 secs between their fastest lap time and their race average lap time. Look at the top drivers in your club - what is their difference and what is yours?
When you make any changes to the car look at that difference as well as your fastest lap. Yes, you need a fast car, but you also need a car you can drive very consistently over the five-minute race. Some changes will make the car fast over one lap, and some will make the car more consistent. You need both!! HTH
