I can't offer any solutions but wanted to let you know you're not alone! ...
I have a similar problem - LD11's switch on/off and dim. However, I thought it might be linked to interference from a neighbour although I have no proof yet. I also don't have an X10 filter but will be fitting the 250A version today or tomorrow. The weird thing here (and leads me to suspect that it may not be a neighbour) is that HomeVision hasn't reported any received X10 signals. I've asked Craig (HV developer) whether the LD11's might be seeing a signal which is too weak for the XM10 to register - no answer yet.
This is unlikely in your case though as you have a filter (providing it's working!)
The other thought I'd had was that the LD11's (40 odd in number) are somehow starting to fail. Mine are 2-3 years old now - how old are yours?
I guess the other posibility is rogue programming - what controller are you using?
Paul.