Thats why I just use the arduino as 'pulse to ASCII' translator
That is a bad idea, especially when you are sending serial data from within an interrupt routine.
You are sending 8 characters (including the linefeed and carriage return) on each interrupt. At 9600 baud it will take over 8 milliseconds to send just one message during which time you will have received at least 48 more interrupts just from one RPM input.
You would be a lot better off counting the pulses in the interrupt routines and then periodically sending a summary to the PC.
Pete