I am using this entry level UHF antenna.

http://www.dicksmith.co.nz/antennas/dick-smith-hd-uhf-outdoor-antenna-pack-dsnz-l4735

DSE told me it would be OK for our location (The Wood, Nelson).  But If the weather is poor - rain or even a lot of cloud on Mt Campbell where the transmitter is, the picture breaks up.  This is worse on Choice, Prime, Maori etc.

Our house is about 40km horizontal distance from the Freeview UHF transmitter, with about 1,200m altitude difference. 

The current antenna has clear line of sight and it pointing right at the transmitter tower (Mt Campbell) which I can see on good day.  The rotation of the antenna is correct.  I have quality RG6 cable and good F-Type crimped, sealed connections.

Advice appreciated.  Specifically should I try a different antenna and what sort?