Speedup poke - why do some Dragons fail

[Serenarian]

Since you have re-awakened my interest in Dragons I have always wondered why some Dragons can't run at the faster speed.
For instance I have two Dragon64s - one with a version 2A board which will run at the higher speed whereas another with a version3 board will not.
Curiously when I say one won't run at the higher speed, that's not quite true. It does indeed run faster (and the RAM checks out when running faster) but the video output flickers and has spurious characters.
Is it down to the 6809 processor, the SAM, the 6847 video chip or the RAM?

[zephyr]

That's normal for all standard CoCo 1/2s and Dragon 32/64s running at double (POKE65497,0) speed. The hardware RAM refresh, and video display are always disabled in this mode.

[Serenarian]

Thank's for that - and the link. I'll try some chip substitutions now then.