intermittent overheating, coolant loss, no leak I can see

Hi all, I have a 2008 Corolla with 277k miles on the 1.8 vvt 1. Here are the details of my question, and thanks very much for insights.
- Head gasket replaced about 10 months ago. Runs great albeit with a rattling noise below 3k rpm that is difficult to diagnose.
- Also new radiator replaced at same time as head gasket.
- I replaced rod connector bearings on lower end, but they were probably fine according to plasti-gauge measurements.
- Recently, the engine overheated due to loss of coolant. I have no idea why the coolant had gotten low.
- There was grayish sludge around radiator cap. Could this be old gasket from before, or RTV in the system from the repair?
- It has lost coolant and temperature has risen 3 times since then.
- But I can not find a leak anywhere.

- The oil is clear and clean, no bubbles or loss of oil in the past 10 months.
- No steam or smoke from exhaust at any time.
- When car is running well, temp gauge on the dash is just below middle of the gauge.
- It takes several days of town driving, then suddenly the temp will go up.
- When overheats, gauge is 80% to the top, but no warning light comes on. Then gauge fluctuates.
- Been getting a code for catalytic converter performance, but car runs great. Takes a couple of days for the code to come back on.
- Recently a bit of rough idle while cold.
- I took off the air intake hose.
- There was some sludge in the throttle body. A gooey syrupy sludge. This goo was also in the vacuum hose that connects up to the valve cover.

IDEAS? Thank you for help!:
- I am probably going to replace the coolant temperature sensor because I am at a loss about how to figure out where my coolant is going.
- Water pump is about 75k miles old. Can it be bad and not even leaking?
- Could Thermostat be the problem?
- Where in the heck is the coolant going?

Thanks much,