I have a K&B outboard I haven't run in two years, the reason is I never ran a full tank through it. It would run about 2 minutes then shut off like its out of gas. I changed tanks and tubing several times any suggestions.
1. As has been suggested, you might need new bearings. If you ran the fuel out of it and then lubed it up well, they should be ok.
2. If you did not remove, dry, and regrease the prop shaft, you will most likely need a new one.
3. If it has a good idle and transition to high speed while on shore, it's too lean. If it sputters and spits out fuel, and you need to keep revving it up to prevent stalling then it's ok. Note, DO NOT keep the engine running at high RPMs out of water, it will throw the rod. However, it's ok, and often necessary, to keep blipping the throttle so it doesn't load up while idling.
4. There's nothing wrong with running 30% or higher nitro. I used 25% on the one I broke in this summer, but ended up switching to 50% so I didn't have to carry 2 different fuel bottles.
5. Are you using a single tank, or a main and header tank? Just to confirm, you are using the exhaust pressure, right?
6. What prop are you running? It is sharpened and balanced, right?
7. You might also try a different glow plug. Nothing wrong with the K&B ones, but I've also had good luck with McCoy.