1999 Chevy Suburban coolant leak

I replaced radiator with new, it started leaking under cap. Replaced cap 3 times, same problem. Returned radiator and installed a new one again. Still leaks under cap. Usually only does this when engine is cooled overnite. Once engine is warm, leaks no more. I have a 16psi cap on it. I took a seal from another cap and put on new cap, thus making rubber seal 2 times the thickness, no longer leaks under cap, now it leaks at hose fitting from radiator to water pump on the radiator side. Not sure what to do next. Is it the plastic expanding and contracting causing this problem or do I try to switch to a lower psi cap, like a 11 or 12 psi. Please help, am going nuts over this. By the way, I am running dex-cool. Thanks for any help, Mark