Looking at that code, it appears to me that there is no 'slush' time at all in the ISR. In the original Renard the ISR is entered once every 30 uS or so, while in the servo code it's entered once every 4uS or so when it's active (it's not active all of the time in the RenServo code). With a 18.432 MHz oscillator the ISR is entered once every 18 instructions, or so, and the ISR takes about that same number of instructions (all of this is approximate off the top of my head). It's not surprising that adding instructions to the ISR would break things. It would have been better to place the instructions for limiting the servo range down in the 'phase0', 'phase1', 'phase2' and 'phase3' code near the end of the assembly file.