Overdubbed tracks play back at different speed (slightly)

I'm using Producer 8 on two different workstations:
Main studio setup
Desktop with Win XP SP3, AMD Athlon 64 X2 2.6GHz, 4GB RAM, Mackie Onyx 1220i via firewire, record/playback on internal RAID 1 harddrive set.  44.1 sample rate, 256 sample buffer (5.8ms latency)
Portable (laptop) setup
Toshiba Satellite, Win 7, Intel i7 1.73GHz, 4GB RAM, Mackie Onyx 1640 w/ firewire thru Siig adapter card, record/playback on external USB 2.0 harddrive.  44.1 sample rate, higher latency (around 20ms or whatever I can set it to get it to stop popping and dropping out).
 
Here's the situation:  I recorded tracks for guitars, vocals, vdrums in the studio to the metronome, then bounced everything to a couple of tracks and saved the song files as bundles to put on the external drive to record acoustic drums at my drummer's house with the portable setup.  I removed all plugins, synths, FX and everything from the bundles first to make sure there was no extra junk to process on the laptop.  Then I opened each bundle on the laptop, saved as a project file with audio files on the external drive (if I record from the opened bundle, I get dropouts like crazy), and proceeded with recording 9 tracks of drums to the music reference tracks and Sonar metronome.
 
Here's the problem:  after recording some drums, we'd play back to have a listen and the drum playback would become slightly out of sync with the reference music tracks and metronome over time (after 30 seconds or so).  This is not a latency perception because I created "latency-compensated" tracks of the original reference tracks that I shifted by those milliseconds caused by the sample buffer, so even when I could get the drums and references to start in sync, they would still eventually diverge.  I was hoping it was a "laptop issue" that wouldn't matter once I imported the drum tracks onto my studio workstation, but I'm getting the same thing.  Somehow the recording and playback clocks on the laptop don't seem to be the same and the resulting drum tracks are all slightly slower.
 
Why would the laptop setup play the reference tracks perfectly in sync with the metronome but the recorded drums would diverge over time?

... and that the sampling rate and bit depth are the same in SONAR and the audio interface.