There are several causes for sinking while cooling:
Ingredients not warm enough or same temperature. Use an instant read thermometer to check temperature of ingredients. Everything should be in the 68-70 degree range. Make sure all ingredients are close to same temperature.
Butter and sugar not properly emulsified. If your not under mixing, then butter may be too cold to properly emulsify.
Be sure you beat the eggs in well after each addition. Also, be sure eggs are not too cold—again 68-70 degrees is optimal. Emulsion is the key—without it your cake will fail.
Too much sugar. Sugar is hygroscopic. It competes with flour to absorb moisture. When heated, sugar will release moisture. Measuring by weight will ensure proper ratio of ingredients.
Under baking. I don’t like the toothpick test. I prefer the doing spring test. Gently press the center of the cake with your finger. If the dent springs right back, the cake is done. If the dent is slow to spring back or does not spring back the cake itnt backed
Too much leavening will cause the cake to rise too rapidly, then sink as it cools.
325 is actually a better temperature for most cakes. Just be sure you oven temperature is actually 325 degrees.