Mercury 4 Stroke 60 HP dropping RPM’s at full throttle

[CeoswaltTX2020]

I have a 4 Stroke Mercury 60 on Sun Tracker Bass Buggy 18 that has less than 100 hours on it. I have had it serviced by boat mechanic. Had oil changed and had everything else checked out with computer diagnosis tool - no issues on computer. I’ve also installed brand new fuel line. I am losing about 5-600 RPM’s after boat is given full throttle, levels out at 5100-5200 RPM’s, then drops to about 45-4600 RPM’s. I’ve also tightened the prop to make sure we didn’t have any play from it being loose. This is so dang frustrating. Any help would be greatly appreciated.