It's always good to check all of the connections after de-winterizing. Start at sea strainer and make sure it's clean and no water leaks. Check the Vdrive cooler and trans coolers for leaks, loose fittings. Heater hoses at tstat, water pump, oil cooler, heater/shower.
Check the block drains and the freeze plugs. I have had a freeze plug (its actually a plate) leak once, I took it off, banged it flat with a hammer and reinstalled with permatex, no probs. They are attached with 2 machine screws.
Any air leak in any of the above components will cause cooling issues, unstable temp readings.