Scott ... I'm going to try to make it down on Sunday. With your motor I would look at afew things.

1) going from 30% to 25% I would use a #6 plug.
2) Going from 30 to 25% will require leaning the motor out by two to three hours and the motor will want to run between 20 to 30 degrees hotter for the same performance. If you were running it at 220... 240 to 260 with the lower nitro.
3) I'm running an RBR3....similar to your 353, and it won't run very well at all until it gets up to about 265 on 25% with a #6 plug.
4) Check the rod for wear. Sometimes connecting rods that are worn will cause a very unreliable idle when up to race temps.
5) I don't know what head clearance you started with when using 30%, but dropping to 25% won't usually require removing a head shim. It will generally require a hotter plug and leaner top end. If your motor wasn't detonating at 30%(dry piston top with sand blasted look) dropping to 25% won't usually require reducing head clearance.
6) I would definately be looking at the fuel tank, and check the rod. Those are the two things that I would look at first. I would definately be running a #6 plug and lean the motor out.