Radiator cracking on model 4 with 912ULS

Hi there, new Kitfox owner here. I picked up my 2006 built model IV about four days ago and have been flying it from Minneapolis back to my home field in California. I’m experiencing some pretty bad issues with the radiator cracking. The first time it happened I landed at McCall ID and noticed a pool of radiator fluid under the plane while doing the pre-flight. I pulled the radiator cowl and discovered the plate on the under side of the radiator had cracked so badly that it could be pulled off with very little pressure. I called Kitfox and they told me they had a spare radiator on the shelf. McCall Aviation were kind enough to lend me a line pilot who flew me down to pick up the new radiator, which I installed on the ramp. After engine testing this morning I flew the plane to Coeur D’elane in northern Idaho. Once again while pre-checking the plane I discovered the same cracks again on my new radiator. The flight from McCall to Coeur D’elane was only two hours. I’m racking my brain trying to work out why this is happening. First thought was the radiator cowl was flapping against the bottom of the radiator due to prop wash, but I flew the stretch to Coeur today without the cowl on. Other ideas are some kind of prop resonance, or some kind of resonance through the frame. Has anyone seen this happen to their model IV radiator?

attached a couple of pics, first broken radiator:

Attachment 22987

Second radiator, cracks appearing after 2 hours:

Attachment 22989
Attachment 22990

Wondering if a resonance is being induced in that bottom plate from the prop wash

Attachment 22991

I don't recall seeing this problem as a topic on the forum. I don't have a Rotax, but here are a couple ideas. Hopefully some of the Rotax people will have some better ideas

Is your radiator mounted with rubber shock mounts? That could help.
Vibration could be the cause. Do you know if the prop has been balanced?

Just a rough guess ... the aluminum radiator is probably going to be much more susceptible to fatigue & cracking than the original copper & steel ones, especially mounted as it is on a Mod. IV. Bracing the bottom of the radiator like it’s done on the newer models might be necessary, if possible.

few years back when I got the replacement cooler mine cracked just like yours. I got another on warrantee and with the new one I took some plastic wire ties and wrapped around the cooler in the center to give some support. about 10 years later it's still good. I feel they should put a piece in the center to joint top to bottom for support. mine broke just like yours.