I just took a look at the 515 datasheet and it seems to be pretty clear that following the stop bit, there MUST be 1 bit time of 0 (for the start bit). Is this happening?

The 515 (like most 8051's) oversamples by 16x the baudrate. The actual bit is sampled on the 7th, 8th, and 9th counter state. There is a lot of good information in the User's Manual, probably more than most 8051 user manuals.

http://www.keil.com/dd/chip/2971.htm

Anyway, I didn't see anything that would cause the problem you are having. There are serial port timing diagrams in the User's Manual that may help.

Jon