Alan, it sounds like one of the following:
1) the signal is not strong enough, and drops out occasionally.
2) Your laptop either has some other process running which intermittently demands most of the available processing power, causing the DTV application to pause (Windows itself most likely)
Without seeing the problem, it could be either of the above. Does the problem go away if you are in a strong signal area? Your DTV application should tell you the current signal strength and signal to noise ratio. Are you using the caravan aerial and booster? If so, then on many sites you may need a better aerial.
Hope that helps a bit.
Cheers
David