PIC Project Help!

[Jaicen_solo]

This is a little off topic I know, but I don't know where else to ask this question.
I'm building this interface: http://www.zimmers.net/anonftp/pub/cbm/documents/projects/interfaces/mouse/Mouse.html#serial

It allows you to use serial mice with the C64 to use with the MSSIAH sequencer, so it is sort of music based  .

I've built it using a PIC 16F84, because I can't get hold of any PIC 16C84, nor do I know if I can program them.
Anyway, the PIC i'm using has a schmitt function on the external interrupt pin, so I tried the workaround detailed on the webpage using the RS232 chip. That didn't work so I took off the mods, however I notice that when I touch pin 6 (RB/int), the interface will work perfectly for 10-20 seconds, and then lock up again till I touch the pin.
This is really doing my head in. I've tried adding caps to ground, and large resistors to ground but nothing works.

Can anybody suggest how I can get the thing to function without my finger poking it every few seconds?