That's a little puzzling. Has it always done that, or is it new? Generically, fluctuation oil temp is usually suspected to be a grounding issue, but the fact that it only hits at a certain temperature is confusing. You might want to give our technical support team a call at 425-402-0433 or support at dynonavionics dot com
# 1 - with any engine sensor or device connected to ANY ems system is to use the system grounds, even if you have to double up on a ground wire. This may not apply in your situation however it is a good general rule IMO & experience.
There may be other causes for your situation but the above is a first ‘port of call’ & most cost effective first step.
# 2 - worst case may be a defective oil vernatherm (thermostat) valve, or something completely different.