melition

i have an sr with bolt ons running with the t25 it came with.
since ive gotten the engine when i go wot it hits 16 psi and starts backfiring.
i adjusted the tps and set the wastegate at 12 psi and it ran perfect at wot.
its been running great since, but today it jumped back to 16 psi and started backfiring again.
i didnt mess the the actuator or anything. its the first time i boosted since it was running perfect.
what would cause it to go back up to 16 psi by itself, and also backfire when it goes past 15 psi?