With digital TV there are two factors - signal strength and signal quality, of which the latter is the more important. A TV with a weak-ish signal but good quality should be a watchable picture; a TV with 100% signal but poor quality may not be.
Your problem looks to be one of the commonest problems - noise or interference. Here at home we have indicated strength of nearly 90% and quality only a few points less, but if a car with poor ignition suppression goes past the picture will freeze and/or pixelate.
On most digital TVs there is some way of displaying the signal strength and quality on screen which may help you assess the situation.
There is also the station selection problem as someone else has said. All digital TVs when tuning will scan from channel 21 to 68 starting at the bottom and store the stations as it finds them. If poor signals are found in the lower numbers and better signals higher up, good TVs (like Sony and Panasonic) will offer you the higher channel numbers as the basic stations and shove the poor duplicates at station number 800 and above. Cheaper TVs will store the first signals they find and store any duplicates at 800 and above irrespective of signal quality. This could be the problem.
The only way to overcome the above - if your TV has the facility - is not to use the automatic installation but rather to input the channel numbers for the area you are in and force it to scan manually.
Don't be confused between Tv and station channel numbers. TV channels are the absolute frequencies: for instance Emley Moor uses 41, 44, 47, and 51 for Ch4, BBC1, ITV, and BBC2 analogue respectively. The digital signals are on channels 40, 43, 46, 49, 50 and 52. Each of the digital TV channels carries a multiplex (or mux) of up to eight TV stations and a number of radios stations as well. So whilst channels 1-5 and stations 1-5 are the same, BBC News for instance is one station within one mux but is number as 80 when you view it.
The TV is not really a TV as such, it is a computer that thinks it is a TV. If you choose to watch BBC News on station 80 it will search the muxes it is receiving and decode the station from the others on the mux that is carrying it.
If you know which transmitter(s) serves the area you are visiting you can find the TV channel numbers carrying the muxes before you leave home at
http://www.ofcom.org.uk/static/reception_advice/dtt_pocket_guide_3_0.pdf
You can find which mux carries which station at
http://www.dtg.org.uk/industry/dtt_channels.html
Note that only main stations will carry all six muxes - most relays will only carry the main three.
If your TV has a manual scan facility you can work out the frequency being used by taking the (signal) channel number, multiply it by 8, and add 306, or
Channel n will be (21 x n) + 306. Emley Moor on channel 40 will be on 40x8=320+306 or 626MHz.
I hope that makes sense.
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