Timing Jumping - Need Help.

I have a 2005 Rhino 660 and am on starting on my 5th rebuild in 6 months (twice by me, then last 2 times at an engine shop). Need Help. Timing keeps jumping. The last 2 times I had an engine shop build it and they obviously are missing something too.

I have $4,000 into PARTS/Labor and need the hemmoraging to stop. The last time, we took everything back to stock parts and got about 3 days out of it (longest yet) but still it jumps timing.

We have replaced the timing chain 3x, the chain tensioner 1x (last time), the cam, the crank, taken to stock CDI box, and replaced the heads 3x.

Anybody have any other suggestions where to start looking? I am going to start tearing down but don't know where else to look.

Thanks.