What I do is turn off interrupts before serial and back on after serial completes. Like this:
Code:
sendudp:
        INTCON.7 = 0
        SerOut2 PORTC.6, 84, [$50] 'ask siteplayer to send udp
        INTCON.7 = 1
        udpcount = 0 
        gosub delay
        return
Somewhere in here there is a list of the commands affected by Instant interrupts. I just turn the interrupts off, execute the command and turn the interrupts back on.
I'm not sure if this is the best way to handle things but it's worked well for me so far.
Hope this helps.