Have you done any work on or around the cooling system recently?
Try ditching the higher pressure cap, topping the system off with coolant, and burping it again. Double check to make sure all your hose connections are tight.
As part of my timing belt change today, I actually upgraded to a Koyo aluminum radiator with Samco hoses and T-bolt clamps because I'm paranoid about plastic end tanks on the stock radiator cracking (apparently it's a common problem on higher mileage cars). My heart sank when I read the title of this thread because I was worried about coolant leaks all day. So far, so good on my end. I'll send some luck your way.
FWIW, I'm still using stock pressure caps and a stock thermostat with my upgraded radiator for now.