To add to what Scott said, it's a good idea to look at the actual 485 signals, and not just the digital side of the 485 driver. The analog waveforms on the bus may help you determine if you have multiple line drivers that are trying to transmit at the same time.
Also, microcontroller delay routines are notoriously inaccurate, especially if there is an RTOS involved. A 100us delay will be *at least* 100us, but can often be much longer depending on how it's implemented. Use the scope like Scott suggested to see the actual timing.
(I'm about to sign off for the night, so we'll talk again tomorrow)
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