/testfor command stops redstone circuit

commandblock with

/testfor @p[x=-3,y=237,z=-2,r=65536]

causes the redstone circuit it is attatched to (conaining a hopper-clock) to stop updating . when the connection to the commandblock is destroyed, after some ticks the circuit (and the clock) start working propperly again. The fps are not dropping.

sometimes it does update spontanously