RPM Issues. Need your opinion please

[Mckillop]

Over the weekend we were running my 25 ZX in a Poker Run. For some reason when I was at WOT, the rpm would occasionally drop from around 4600rpm down to 4000rpm and then after a while it would pick back up slowly. I could feel the boat slow down when this would happen, so I know its not a gauge issue. This boat with this prop usually runs right around 5000rpm with that prop which is also strange. I did have some cup added to the prop, but that would not have slowed me down I don't think? The boat was also running about 4-6 gps mph slower than usuall as well.
I could have been over trimming it because I need to replace the trim senders and I am trimming based on the feel, but if I was over trimming it would have raised the rpm I would think? Does this sound like it could be a fuel or fuel filter issue? I was running fresh non-ethanol fuel so I don't think it would be the fuel, but possibly the filter? All my other gauges were reading normal.
Any thoughts?

[Ghost]

I imagine it *could* be the filter, but don't know. Couple questions:

How old is the filter? (In operating hours if you know, in months if not).

Carb or FI?

[Mckillop]

Its EFI. I bought the boat in April and was told it was serviced when it was winterized the previous fall.

[mrfixxall]

its a combination of things,,water temps are up and so is the humidity.this will chop rpm's and speed..i will loose 600 rpm's on hot days..

[Mckillop]

It really was not all that hot. Low to mid 80s max. However it was at a hight elevation.

[Ghost]

It really was not all that hot. Low to mid 80s max. However it was at a hight elevation.

How much elevation? Or where was it, if you don't know the elevation?

[Mckillop]

Lake Chelan, wa. Maybe 1200 feet

[Ghost]

I take it you usually run at sea level.

Chelan says 1100 feet. I don't know how accurate this formula is, but I see a lot of online estimates of 3% horsepower loss per thousand feet of elevation. So, maybe you were down about 3%.

But what's that, 10 -15 horsepower loss? While I'm sure you'd notice it, I don't think it explains all of the 4-6 mph you lost. And the intermittent stuff (slowing down, picking up slowly) also sounds like something else isn't happy.