1997 Mariner 150 surging at high RPM for first 20 minutes

[counry_bunkin]

I am having an issue with my 150 Mariner where at high RPM's 3500 and above when first running the RPMs will drop to 2500 - 3000. Sometimes I can get the RPMs to come back up right away by pushing the throttle all the way open but when I bring it back to cruising speed they will drop down again. The other part of this is that it will surge up and down without touching the throttle almost like there is water or debris in the fuel. This has been happening over the past couple of years so I do not think that there is water or debris in the fuel. I have replaced the plugs, filters and hoses and the primer ball is hard the entire time this is happening. The issue clears itself after about 15 - 20 minutes of running and does not occur again until the motor has been powered off for an extended period of time e.g. hours. I have two motors both pulling from the same tank and the second motor does not have this issue. Any help would be appreciated.