Are you using serial transmit interrupts?  If you are you must be very
careful to properly save and restore the state in the interrupt service
routine.  We had similar problems before and that was our cause.  We also
went to a polled transmit routine and used the interrupts only on the receive
of a character as our application was not required to do anything else during
the transmission time.

Another thing to consider...are you using proper level shifting circuits to
get the +/-5 to +/- 12 volts required for RS-232 (as much discussed
recently)?  If not you may be on the hairy edge of your receiver's noise
margins.

Good Luck!!

Mark A. Corio
Rochester MicroSystems, Inc.
200 Buell Road, Suite 9
Rochester, NY  14624
Tel:  (716) 328-5850
Fax:  (716) 328-1144
e-mail:  Mcorio@aol.com

***** Designing Electronics For Research & Industry *****