ditto here too. Having some of the same troubles with Ai Servo mode. One thing I've noticed is mostly when using Canon battery grip, make sure batteries are fully charged when you start and the connection to the body is tight and clean. I'm using a 70-200 2.8L II and a 70-300L and occasionally a 200 f2.8L prime. Not too pleased with this latest firmware update.
5D3, 6D, 1D4, 16-35 2.8L ll, 50 1.2L, 50 1.4, 24-70 2.8L II, 135 F2.0L, 200 F2.8L ll, 70-200 2.8L II, 70-300L, 100-400L, 24 TSE L II, 85 1.2L II, 100 2.8L, ZEISS 100 2.0, ZE 50 2.0, ZE 35 2.0, ROKONON 35 1.4, SIGMA 35mm f1.4, SIGMA 85 f1.4, S110, S100, sony RX-100 and one big-a$$ bag