The set in your dining room has lower 'sensitivity' (i.e. the ability to handle weak signals) than the one in the kitchen. You might need a better aerial but (given that the existing signal strength seems to be nearly good enough for the 'weaker' set to do its job) a cheap signal booster (about £8 from Asda) will probably solve the problem.
For further details about why only certain channels are affected, and only at certain times, please read my posts here:
http://www.theanswerb...y/Question858245.html
Chris