Dell Serial Ports

I have a piece of hardware that uses a specialized serial adapter in order to communicate to a PC. The adapter has a 9-pin serial connector. I have used this adapter on 3 different Dell PCs: a Optiplex GX270, a Dimension 4700, and a Latitude D520 laptop. All three have built in serial ports. All three are running Windows XP SP3 with the latest updates. All 3 are fresh loads of XP with no other software installed except the software that communicates between the PC and the hardware device.

Neither the Optiplex or Dimension will communicate correctly to the hardware device. The Latitude laptop works fine. With the desktop PCs, it seems to communicate intermittently. Some functions work, some do not. The adapter is powered via the RTS and DTR signals (they have to be on). The connection settings are 9600-8-N-1.

The software is configured the same on all three machines. I've checked the bios on all three and the serial ports are enabled.  I've re-installed drivers with no change. 

I'm stumped. Are the serial ports implemented differently in the desktop units? Anyone have any thoughts on what might be going on here? Anything I can try?

Thanks.