wspr - sdr1000

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wspr - sdr1000

This is a question for people running WSPR on an SDR-1000.
I wanted to run a second WSPR tranceiver with an SDR-1000 I have available. It is nice because apart from many features there is a panascope where you have a real time visual display of incoming signals.
So far I have this tranceiver working with excellent performance. The only problem I could not solve so far is as follows: I start the program and the mean DT between the first 10 stations received is approx zero. After a few hours I see the average DT going to +2 seconds, say 8 hours later all signals show like +2.8 or more. When I stop WSPR and start it again, the DT difference is still there and keeps on getting larger until allmost no signals get decoded ( 12 hrs or more ).
I have various computers running and they are time-synced by different ways like 2 GPS receivers, NMEATime, Meinberg etc... . They all show the same time within milliseconds to the clock on the WSPR program.
The same thing happens with WSPR 2.0 and WSPR 2.11
I noticed that when I when I go to the SDR program settings: setup/audio/VAC/latency, and I make a change there, say from manual 100 to 101, WSPR is displaying the correct DT figures from then on, but starts to drift away again in the same manner as before.
I use Windows XP sp3, PowerSDR 1.18.6, virtualcable 4.03 or 4.09.
In the mean time I made a hardware workaround, using a second soundcard on receive, instead of the virtual cable, DT is now stable. Still using virtual cable on transmit because the spectrum is cleaner. Hopefully there is no DT runaway on transmit. Would be glad if anybody could comment on this.
By the way I found DL6NL one of the best time synced stations on 20m, when my station is running correct DL6NL is allways whithin 0.1 DT.
Greetings, Mark.