I have a 98 Discovery in the shop that will barely run. When you get it started it pops back through the intake like an engine would do with a rounded cam lobe or stopped up exhaust. I removed the valve covers and all rockers are moving and I disconnected the exhaust down pipes and no difference. I have codes P0307 P0308 P1340. Could the camshaft position sensor cause this or maybe camshaft jumped timing or am I missing something simple.

 

Have you check the engine compression? That would narrow it down.

 

Yes the compression is good

 

Worn cam lobe is not unheard of.

 

How many miles are on it?
Check the timing, I'm betting the timing chain jumped a tooth or is stretched.