This could be due to a few reasons, so please check the following;

• What is your batch size? This G70 system is designed for brewing larger batches. The 70 L (15.4 US Gal) maximum pre boil volume and 17 kg (37.5 lb) maximum grain bill, allow you to make enough wort or beer of any style as a pilot test batch or to share with your friends/homebrew club.
  With the standard pipework, a minimum batch size volume of 40L and an 8kg grain bill is required to ensure the overflow pipework is engaged during mashing which allows for the most accurate and stable mash temperature management.
  With the micro pipework, a minimum batch size of 30 L (6.6 US Gal) and 5.5 kg (12.1 lb) grain bill is required for optimum function. For batches smaller than these, our G30 Brewing System is the perfect size for batches up to 28L (6.2 US Gal) pre boil.
• Is the overflow pipework engaged? Ensure the overflow pipework is engaged during mashing which allows for the most accurate and stable mash temperature management.
• Did you use the strike temperature feature for the G70? With the G70 there is a large proportion of room temperature grains being added to the mash water which can cause the temperature to swing when just using the first mash temperature and this swinging in temperature can occur for an extended portion of the mash.
• What is your grain crush like? If it is on the fine side and the overflow pipework is not engaged then the slow flow of wort may result in the PID algorithm not being as efficient resulting in some temperature fluctuations.