I'm having a similar problem using the tinybootloader, but when I use microcode studio's bootloader I don't have any issues.

When using the tiny loader the first time I bootload my program to the chip it works, but I get the following error in the Tinybootloader window:
----------------------
HEX: 9 hours old, INHX32,16Fcode+cfg+eeprom , total=14546 bytes.
Searching for PIC ...
Found:16F 876A/877A
WARNING: PCLATH not initialised before GOTO! ... sucessfully repaired.,
WRITE OK. time:9.203 sec
----------------------

The problem I am having is I can't bootload a new hex file to the chip after this, everytime I try I get the following error:

----------------------
Searching for PIC ...
pic sending unknown data: "K" <> E0h=à
Check baudrate & Start Write while PIC is not sending serial data (e.g. in reset)
ERROR!
---------------------

I would appreciate any advice you may have.

Thanks