Right, I think I have sorted this... I took today off (primarily to get my new iPad!) and ended up spending the day trying allsorts of combos.. Results are :-
TBS6984 (latest 1.0.2.1 driver) + TBS 6981 (latest 2.0.1.3 driver) = continuity errors
TBS6984 (removed from system) + TBS6981 (latest 2.0.1.3 driver) = working
TBS6984 (latest 1.0.2.1 driver) + TBS 6981 (removed from system) = continuity errors
TBS6984 (1.0.1.8 driver) + TBS 6981 (removed from system) = working
TBS6984 (1.0.1.8 driver) + TBS 6981 (2.0.0.20 driver) = working!
So the upshot seems to be that although the TBS6981 will work on it's own just fine with the latest 2.0.1.3 driver, if you have both cards in your system then you need to roll back BOTH drivers.. No idea why this is, but it seems to be the solution.
Early days but it seems fixed at the moment...
what channel change times are you getting say between FTA and Encrypted and between encrypted and encrypted?