Help needed from the outboard motor mechanics on here

Quick rundown of my current situation. I bought a used boat about 3 months ago, boat has a 1998 mercury 175 w/EFI. The person I bought the boat just recently had a new power head put on the motor and had it serviced at the same time by a reputable mechanic. The first time I took the boat out I had power issues, the boat bogged down, would not get on plane and would not go faster than approximately 5mph even at full throttle. Took the boat back to the mechanic, and he said it was something to do with the fuel pump and fixed it at no cost. Took the boat out on memorial day and it ran like a top, so I assumed the problem was in deed fixed. however over the weekend I found myself right back where i started with this boat.

The issue I am having is, when I throttle down, the motor the bogs, the RPM's go to the 3500-4000 range, and the boat does not get above 5 mph or so. But after a while (10-15 min or so) of riding with the throttle wide open the boat slowly begins to pick up speed and gets on plane. but if I stop or even slow down, I am back to square one. Does anyone know what may be causing this? I am pretty good at fixing things myself but only if I know what I need to fix. If worst comes to worst I will take it to another mechanic, so if no one knows what could be causing this maybe someone could reccomend a good mechanic around Prairieville.