Interesting ' problem' with my Tascam DP-02

[TascamJimi]

Hey folks, I've come here with some really silly questions..... that have been answered and not laughed at so....going to try another one....and this is definitely a strange one...

I recently recorded an arpegiated ( sp?) guitar line that last about 4 minutes....upon playback, at about the 2 minute mark, it starts to slow and go out of tune... sounds exactly like batteries dying in a tape deck...very strange..... as a test, on another track I recorded the same pattern, just using a looper pedal instead of playing the part straight through to eliminate poor picking technique the longer I played......... it does the same thing...starts out fine... then all of a sudden it starts ' dragging' and going out of tune with itself. I know that technically this is impossible with digital..... (or is it ?. )......... but my ears are telling me otherwise.... no modulation effect on the guitar...just bone dry....so I ruled that out....and it's just a single guitar line ...not doubled or anything...
Has anyone ever experienced this ? I have a 5 track sub-mix on tracks 7/8 and they play fine, all the way through.... I can't figure this one out...

 

[Slouching Raymond]

The timing will all come from a quartz crystal, so it ought to be accurate.
I was once told that over a long time they are accurate, but can wobble about a bit.
They are actually electrical/mechanical oscillators, and may operate in more than one mode due to geometry.
The only other thing I can think of is how the sampling frequency gets divided down.
It may involve spikes instead of square waves, which may not register at times.