I'm thinking that's not a Tamiya issue, but a Spektrum one.  I run all my Tamiya full option tanks on Spektrums, and have a few Dx6is.  You might verify your DX6i is'nt on the recall list.  DX6i's have had a couple recalls.   Check Horizon's website for details.

If yours s a non recall unit, then you might try changing out the rx to eliminate the rx as a culprit.  I've started using some of the new cheapo Hobbyking orange rxs and they work fine

Hope this helps.