Not an answer but a couple of ideas (I have same card)...
- have you installed latest TBS driver? They recently changed from using IRQs
- what is the CPU load watching 4 uncrypted HD channels and have you verified how much it increases per active coded HD channel?
Win 7 64-bit SP1, Intel i5-3570K 3,4GHz HD 4000 Graphics, 6GB, TBS 6984 S2 Quad,TBS 6280 T2 Dual
DVBLink 5.5.0 Connect & TVSource (mc_recorder), CD S Thor 0.8W,FTA Aastra 19.2E, S FTA T2, ACAMD 0.6.2
Clients iPhone 4S 5S 6S, iPad2 iOS 9,Win 7 PCs