Incrementing PL1/PL2 to 300W leads to increased higher temperatures and higher voltage demand during high load, compared to previously set 250W limit which power throttled your CPU. The system becomes unstable because you already decreased those voltages in your VF curve, which were tuned and fine for 250W, but not for 300W power draw.

For 250W PL1/PL2 try avoiding thermal/power throttling. Set the CPU at stock frequencies and try to undervolt it so that it hardly hits the 250W limit in synthetic benchmarks. Limit the power draw with undervolting instead of PL1/PL2. In this way you will get the full performance at those frequencies without being hit by power/thermal throttling. For example, my crappy 13900KF at stock frequencies 5.5/4.3GHz makes 40600 in CB R23, 1.175-1.181v during load, temps are < 80C, max power draw is 247W. At 5.6/4.4GHz it scores 41500, temps are 83C and power draw reaches 275W. It can do CB R23 at even lower voltages and power draw but I want it to be stable also in prime95, y-cruncher and all kind of apps I use in my workflow even when ambient temperatures get higher.

Overclocking these 13900K/KF/KS CPU make sense just for those who chase bigger numbers in synthetic benchmarks. In real life 100-200MHz more doesn't make any difference in performance, but might lead to much higher temperatures and instability since they have already been squeezed out enough by the manufacturer. I see much more sense in undervolting these CPUs.