Who makes the toughest/strongest chain ring bolts?

I've got Shimano Ultegra gruppa, but pretty continually break these bolts....I'm an engineer, and today had all the measuring equipment out trying to figure this out, and it seems the bolts are stretching, and then breaking.

Scenario goes like this: biking along, get a ticktick happening from the crankset. Re-torque bolts, sure enough there is one or two loose. Ride more, the tick comes back, go to re-torque, and the bolt breaks. Turns out the bolts are not actually loosening off, but are instead stretching. You can only re-torque them a couple times before the stretch affects the strength and then they break.

Shimano manual calls for 106-141inlb on these bolts, I've been doing them to 125 inlb. Less seems to give me a ticktick. I tried going to max and the bolts broke right away.

So ya, I need something stronger. Even if the bolts went from using a #30torx to #25torx, that would add a lot of strength too! I'm pretty hard on equipment overall...LBS loves me

Matt