Once in a while you created some effect that just goes too quick when you update it every frame. The option is to have a check in order to skip your routine every other frame. Put this inside your IRQ.
start: inc skipper+1 // increase the check byte skipper: lda #$00 // check byte and #$01 // check first bit (even or odd number?) beq passroutine // skip routine on even numbers routine: // routine that will be executed every other frame passroutine: // the rest, executed every frame