As I have described earlier in this list I had simlar problems, and
played with both the PLL power and the PLL divider (to get faster
feedback to the PLL) and could improve the situation, but never got
it really good. It mostly seems to have been because I had a lousy
12V feed, I ended up replacing the power supply and moved a 12V fan
which was connected to the mother board to a separate connector.
If you want to get more info about playing with your tuner you
are welcome to contact me, but I'd first check the power supply.
I still have another interresting problem though, which I as far
as I know am the only one to have:
I need to run an older firmware in the SP8870 than any other
person I have spoken with, or I get serious disturbances,
shown as intermitten higher BER every 2-5 seconds with for
example tzap on one of four channels, and the same problem but
maybe only every 10 seconds and not as bad on another channel.
On the two last frequencies it doesn't matter which firmware I run.
Also sounds familiar but only on the same channels that also have
"decoding" problems.