Well Hazel, this one ain't gonna` be easy.

Tumbling/poor accuracy could be caused by a bullet/sabot combination that's too loose in the bore.

Another possibility is that those Pyrodex pellets are moisture/humidity/age compromised.

As Cayugad said, maybe that 150 grain load is the problem. A lot (most?) guns don't shoot well with that load. If things don't improve with two pellets I would seriously consider investing in a canister of loose Pyrodex RS or Triple Seven and play with 80/90/100/110 grain loads.

Maybe your gun just does not like 200 grain bullets.

Are you shooting with a scope or iron sights? Is everything in the sight system tight. Are you shooting with a good solid rest? Are you shooting at least three shots without adjusting the sights?

See, I told you it wasn't going to be easy.