I had these exact same issues with the temp probes (I have 2 as well - one in the fuge, one in the skimmer section). They would randomly read some large negative value.
Initially I thought as Bigtrout did and started with double checking my connections with some continuity testing, then shortening the cables to minimize the run to the probe. These probes have sheathing, so i also tried grounding the sheathing. None of these things worked.
I've been testing a new temp probe for the last week and it is rock solid, while the old probe is still spiking once or twice a day. Below are the probes - both good and bad.
(Old) Flakey Probe =
DROK 2pc -55 to 125 Deg C, -67 to 257 Deg F Indoor Outdoor Water Cooling Heater Temperature Sensor, 3 Meter Waterproof Cable with Stainless Steel Probe This is the orange color cabled temp probes - I actually cut the wire down to 1 meter from PI enclosure to Sump and coated probe in silicone.
(New) Solid Probe =
Diymore 5Pcs DS18B20 Length 2M Waterproof Temperature Sensor Thermistor Temperature Controller Thermometer with Probe (2M) - Black probe - again I cut the wire down to 1 meter and did not coat in silicone.