I'm going to try and take a stab at this....no promises.
1.) If your Evinrude 18hp is a short shaft....and you have it on a transom that is 20 inches or greater, the propeller may be up a little high and causing some cavitation when you peak at planing out and losing some acceleration. The anti-cavitation plate should be located just below bottom of the boat.
2.) If your propeller is damage or bent, it could be causing some cavitation and causing you to lose some acceleration when your planing out.
3.) Something is dragging on the boat.....trolling motor not lifted out of the water enough.
Hope this helps!