Please check my thinking here

I just upgraded the CPU on a customer's socket 775 computer from an E6700 to a QX6850. This machine is used for graphics design in a printing business. The QX6850 is on the compatible processor list for this motherboard and the motherboard bios is A06 which is the last one issued for this board.

Here's the issue. Oftentimes when the computer is started after a power down it will not boot unless I power it off again and push the power button a second time. Then it starts to boot alright but during post up I get an error message that says something about the bus speed being incorrect and "press F2 to enter setup or F1 to continue." This is a Dell mass produced computer so there are no options for changing frequencies in the bios. Pressing F2 to enter setup is fruitless. In the bios sys info tab the CPU is recognized correctly. If I press F1 to continue it boots right up into Windows and the computer operates perfectly normal. CPU-z shows the bus speeds of the CPU and memory to be what they should be.

Am I probably correct in assuming there is a non fatal glitch somewhere in the bios microcode?

I had an HP that had a similar problem (seems like I am always looking back at that rig when someone mentions microcode!) with a cpu update. Long story short is that it was exactly what you think

non fatal glitch somewhere in the bios microcode

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and there was no fix. The only reason I mention it is that along with this glitch was a secndary problem. When I checked CPU-Z I noticed that I only had half of the rated speed of the chip. The only way to overcome it was with a software workaround -- so be sure to double check that you are getting the full spec from that chip in CPU-Z

I would also suggest that you try reflashing the BIOS, though it's a long shot. Could have simply been a slightly corrupt update.