Duffffin,
I see drift in the bsp when using external tempo sources and have formed the opinion that it is most likely one of these:
A variation in the tempo producing computers latency due to resources being consumed by other programs which come and go.
A second or maybe even more tempo sources in the same rig, Its easy to have send tempo Active in more than one app, program, hardware.
If the tempo comes from a device via wireless then a router bottleneck or some other sort of wireless interruption can drop packets of data and cause wild chaos until the data flow stabilises. Read Ableton link and an old router.
If the tempo comes to the bsp via midi direct from another piece of hardware then this should be pretty stable unless that other hardware is not the generator of the tempo but forwarding it, then whatever's up the chain from there is suspect. Also there could be a tempo rebroadcast in the midi network which has a lag and gets in the way on top of the main tempo.
Did you change something in your rig, add something, take something away, adjust preferences?
My vote is on a change you made deliberately or inadvertently.
Personally i don't trust my DAW to be the tempo master ( it's the computer that's the problem) and would rather choose Midi to broadcast tempo, away from wireless and computers with resources issues.
