Get them to change the LNBF on the dish is sound like a Local Oscillator drift issue. (10.7GHZ L.O.)
For the Satellite digital QPSK service they use a PLL LNBF (Phase Lock Loop).
If it is cactus it will do what you have described.
Get them to actually get the ladder of their van and climb on the roof and do something.
I am a Satellite Engineer not an installer and controlled this service work a number of years ago for Foxtel & Austar.
The Installers are underpaid in most cases and will do as little as possible where they can.
Band aid approach is their motto.
Hope this helps.
pce229073