Originally posted by flight risk
I have tried leaning out the engine as far as I can without dying.
That is likely your problem, and is the fastest way to ruin an engine. Richen it up. The reason it dies after 5 minutes is that you're running it too lean and it overheats. I see folks at the field do this nearly every day I fly, and they swear up and down at their engine when in fact they're just not running it as they should be.
Do you have any experienced help available? Glow engines take some getting used to.