P38 Braking in an odd way

I have noticed that sometimes when braking the car feels as though one of the discs are warped. The fronts were replaced a couple of months ago so should be OK. What is odd is that this feeling isnt always present, sometimes I can brake with no shudder/jerking at all so this implies the discs are OK. My ABS pump seems to be more active too in that you can hear it after every 2nd of 3rd brake press, not sure that right. I am not losing any fluid and all the brake pipes are fine, I had front to back replaced a month ago, so I dont suspect it's an air in the system issue. I know that this is a complicated set-up so am looking for any pointers, would a sticking caliper produce this effect from time to time?
4.6 HSE 1998 Model

does the pedal feel like its sticky? by this I mean it has resistance and is then notchy

No the pedal doesnt suffer the clicking I have read about. It varies from being firm almost from when you put you foot on it to soft and travelling about half way

faulty accumulator possibly