1Gig | Getting Sub 300Mbps intermittent, low of 108Mbps

Hi, 

Paying for a 1Gig package, getting less than 300Mbps with lows of 108Mbps and less. 
Already had a new router installed, that fixed the issue for a while, however, it has now returned and its now really starting to annoy me. I am told i cannot get a bill reduction as its within the minimum set parameters of the contract, but this is simply not acceptable anymore. If im paying for a package i expect the listed speeds, not 1/10th of the speed.

How can this be sorted?