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Are you in AF-S or AF-C mode?
What are a1 and a2 set at? Focus, release or focus+release?
Are you using single point, dynamic AF, or Auto AF?
AF-S, Focus Priority, Single Point.
I went to my Nikon 50 1.8 and could not get the problem to repeat in about 100 tests. Maybe it is the Tamron lens not properly communicating with the D-700? I'll do some more testing.