Take a look here as these guys advice has helped me in the past. Leads and cables would be my first place to look. If you use SCART cables I would swap the cable and if you can use HDMI I would use that but depends on the age of your Sky box and TV. http://www.satcure.co.uk/tech/interf.htm
I've repaired my monitor a few times - 50p worth of capacitors. When I go to the tip and see pallets full of old monitors, I know 90% of them will be fixable for pennies... Makes me sad.
You can't really complain about her buying a new telly after you launched HMS Axminster recently. :-D
IF using a scart lead, Double check your scart lead. Could still be the sky box but theres a good chance its the lead.
RGB is far superior to PAL when it works as it should.
PAL sends all the colours down one wire (pin) of the scart lead, RGB down 3 separate ones, one for each colour, so its common for one colour to drop out due to an iffy connection.
That said, if your Sky box doesn't have an HDMI connection (better picture again), its due for renewal anyway
Also, if you need a new box and just have the basic free channels a Freesat box will work with your existing dish.