Ideally you should get a decent aerial fitter to install something like this:
http://www.maplin.co....e.aspx?ModuleNo=22523
or, if you're in a lousy reception area, something like this:
http://www.maplin.co.....aspx?ModuleNo=221124
together with some good quality, low loss cabling.
However, since your signal strength is nearly good enough, a simple signal booster (£9 from Asda) might well fix your problem.
It's also worth remembering that, in areas where analogue transmitters are still switched on, all Freeview signals are currently being transmitted at reduced power. (That's necessary in order to avoid interference to the analogue signals). When the changeover is completed you'll get stronger signals, with far less likelihood of any sporadic reception problems.
Chris