Same problem here with 18-270 and D90 ...and it happens frequently:
-when focal lenght is above 200mm (but also at other focal lenghts)
-when I'm focusing very close objects (50cm)
-trying to focus in dim light.
Sometimes I fix the problem going to MF and back to AF or switching D90 off and ON, but many times this is not sufficient and I have to unlock the lens and lock it back. At this poit the D90 recognizes it correctly...until the next failure.
Are there any other solutions to fix the problem or the only solution is to send it to Tamron...waiting about 2 months?
Have contact points to be cleaned/realigned?