Power loss/holding back when cold

[EpsteintheSwami]

Hi there chaps,
Got a small issue with my K75 that I thought some of you more educated individuals might be able to shed some light on.  Starting the bike up at 4am as I do every morning I ride it away straight away so I don't piss the neighbours off, as soon as I've turned onto the main road I turn the choke off so I don't forget about it (and I won't need it now as the bike will be warmed up by the next time I want it to idle) once Ive done this it seems to develop a flat spot at around 5000rpm briefly before getting past it.  Also after changing up a gear it seems to hold back, like its momentarily short of fuel before accelerating like normal.  Once warmed up the bike runs absolutely fine, although it seems to really lack power below 2000rpm, not sure if this is normal for these bikes or not.
Feels like it  might be caused by an air leak in the fuel supply but I don't know why this would only show up when the engines cold. 
Also, is it normal for the petrol tank to make a loud hissing sound?
Thanks for reading, I await your replies with eagerness!!!

[Scott_]

2 ideas come to my mind. The noise in the fuel tank could possibly be the pump itself, or the vent could be restricted. How much fuel do you have in the tank.
The cold running issue could possibly be due to a engine water temp sensor starting to fail, causing the jetronic controller to think the engine is warmer than it actually is.

Check and clean the connectors for the water temp sensor(behind the radiator on the top of the motor) and the multi-pin connector at the jetronic controller. It may not help, but it's a good place to start.

[The Mighty Gryphon]

Could that noise in the tank possibly be the fuel return? 

Does your fuel pump strainer have the connection for the return line to help prevent pump cavitation?

[EpsteintheSwami]

Thanks for the replies guys!!!
I think the hissing may just be the noise that the fuel pump makes, as it increases as you rev the engine.  When you ask if the vent is restricted do you mean the vent in the tank cap?  Level of fuel in the tank seems to have no effect on the power loss symptoms or the hissing
I will check the connections on the water temp sensor as suggested, I think some or all of the fairing will have to be removed to gain access to it which should be fun. 
What exactly is pump cavitation?
Thanks!

[Elipten]

I would not think the fuel pump would change sound related to engine rpm, as the fuel pump is electric.