I have not studied the problem but I will put in a couple of things.
1- ON INTERRUPT is not the best for time sensitive operations, ASM types are what you need. This is because ON INTs will not happen instantaneously, they will wait for other things to finish.

2- No matter what type of interrupt you use, you will want to get in and out of the ISR as quick as possible. Set a flag or set a AR while in the ISR and act on that in the main routine.

ASM interrupts are a bit difficult at times so Darrel gave us Instant Ints.
http://www.picbasic.co.uk/forum/show...ant+interrupts