What's a real good arm for a Decca Cartridge?

dhcod, not weird at all. With the heavy shelf your wall shelf was resonating at about the same frequency as the Decca/SME combination. Whenever the right frequency played the stylus was bouncing around in the groove.

Even the very best examples have trouble with the heaviest passages. Just get an Ortofon test disc and try out the vertical tracking band. I have done this with the London Reference and it could not do it. 
It is however a wonderful sounding cartridge and many people do not have that many albums that are that difficult. If you are a Metal fan and like Telarc's 1812 Overature you would be better off with a different cartridge. Decca cartridges are also notoriously unreliable. Perhaps there is a problem with dhcod's example but unless he hit the bearing assembly of the SME with a hammer it is unlikely to be the source of the problem. The SME can be set up for any mid to low compliance cartridge by adding enough mass hopefully using a test record to check the results. Oh, I happen to like Poppycock.

+1 bdp24
There is nothing wrong with your arm. It is the cartridge, just one of the things you have to live with when you get a Decca. They can sound great but they do not track so hot. Just like any British sports car.  Most people I know with Deccas have other cartridges they can use on tougher disc. Instead of getting a new arm jest get another cartridge!

Mike