Power issues

[Salty Dawg]

Have 98 250 ox66, on accel sounds like it's sluggish to accel, at full throttle & trim all the way up it will reach about 4k rpm's, then you can tap the trim switch up & then the rpm's will go up to 5200, then a few minutes they will start to drop again, tap trim switch & jumps back up, first thought it was a fuel issue but wth rpm's changing with trim switch has me stumped, tried new batteries as they were weak but no change.