Editing Question - Interpolation?

[SteveS]

I've had a problem lately which I've not had before but when re-sizing in either GIMP or Irfanview, my 800 width images appear soft and out of focus, whilst at aroudn 1300 to 1800 wide they are clear and sharp. I have a feeling something is losing quality in the resize process but can't pin point it. I think it may be a feature on GIMP called 'Interpolation', of which there are four choices - None, Linear, Cubic, Sinc (Lanczos3). Can one of you editing guru's explain what on earth that means - I've always done it on Cubic, but recently, it's just not doing the trick?

Steve