Help with heater leak - only when heater is on

Anyone have any ideas on this?

My heater was leaking. Core was bad. Bought a new heater core on skidim.com. Installed it. Very easy. Everything was working great - no leaks.

Then, believe it or not, the kids were really cold this past Wednesday. Had a mini-cold snap here in Tennessee. Ran the heater a bit - and it started leaking. I thought - I can't have a problem with the hook-up because it worked for 10 hours of use without leaking. I thought - heater core must have gone bad. Now this - next 2 days I run the boat and never turn on the heater - and - no leaks.

So, it makes absolutely no sense to me, but my new heater only leaks when the heater is on. Water runs through it the whole time, so what gives? The old one leaked all the time. Any thoughts?