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

The modern London cartridges made by John Wright have much better quality control than the original Decca Special Products output (which was indeed a bit of a lottery).
I have had Decca for 40 years now, and they will track most records. No they won't do the cannons on the Telarc 1812 Overture, but very few cartridges will track that.
Traditionally they were best mated to damped unipivots. The lack of vertical compliance meant that the vertical vector energy could be dissipated in the unipivot without affecting the cartridge performance. I've used my Maroon, Garrott Bros Gold and FFSS MkIV C4E (John Wright rebuilt with fine line tip) in my Hadcock 228 and Schroeder tonearms (Model 2 carbon and Reference) with great results.

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.

Thanks for all the info! I've got a test record on the way and I'll add even more mass to my Fidelity Research headshell. I've been lining the inside of the headshell with the lead tape that is normally used for golf and tennis equipment. That has allowed me to use the extra balancing weight of the M2-12R. I've been noticing since I posted originally that the mistracking happens almost always at the same place, about 2/3 into an LP. I wonder if I have to readdress my alignment?

If your alignment is *close* to correct it is likely not the culprit. How much does your 'dressed' headshell weigh now?

I would guess around 26g.

Here's a weird one. I swapped out the 3" maple shelf I had installed on my wall shelf just a month ago and went back to the 1/2" maple shelf I was using before and it has only misracked once since then in two days of listening. Either the rigidness of the thicker shelf was bad or somehow there was something wrong about how the table was sitting on the thicker shelf, even though I checked and rechecked the leveling.