My boat has twin T6-354 engines with which I am on a rather intimate basis with. To take a stab at your problem, I would guess the following: 1. I would bet that your engines are feeling a bit of fuel starvation for some reason. 2. If you ever want to simulate this, just forget to open your fuel line valves after a filter change, and you will see the effect, the engines will start going faster and faster (scary the first time) till they die of complete starvation. (Not that I would ever do this). 3. The starvation can be created by an either/or situation. These are (a) a small air leak at one of the plumbing joints, (b) something beginning to clog the pickup pipe inside the fuel tank, or (c) a clogged filter, either the primarly (probably a Racor) or the on-engine final filter. 4. The fact that it does not do it at idle kind of corroborates this hypothesis, since the flow requirement at idle is minimal. 5. Often, it is a combination of a partly clogged Racor primary filter (which is upstream of the lift-pump), or someone who put in a Racor with a 2 micron filter media in the primary filter holder. Perkins do not seem to like these. This makes the lift pump have to exert some suction to draw fuel and this suction tends to pull in micro-bubbles at an otherwise sound fitting (i.e. it does not leak out at rest). I would first of all tighten any fuel line fittings (check for looseness) and then change the primary Racor filter with a 10 micron or a 30 micron and check to see if the problem still exists. If it's gone, you win. If it remains, I would then change the on-engine final fuel filter, and again check. Hope this helps. Let us know what finally resolves the problem. All the best, Ken Bloomfield. IN REPLY TO: "On the way from Atlantic City to Manasquan. The 6-354 Perkins diesel has started to surge. Appears random. Goes up about 150 RPM then right back down. Wondered if anyone has any ideas. Checked Transmission oil and it is good. Seems to only do it when not idling"