I know it's a long debate and some have success, some not, but i never suggest to use internal osc for reliable serial comm. BUT you can reduce the baudrate, say 2400 baud, and do some tests.

Use PICMultiCalc to get the USART settings for you.
http://www.mister-e.org/pages/utilitiespag.html

Yours seems to be the good one... just post your code here and maybe we could suggest you few things.