If you have quite a few or heavy load FX/VI's it could be the issue, seems to me the reason you can get 8 tracks is that each track is handled by a separate core (Quad core)
and if you're taxing each core with each track once you add another track it's too much for the processor to handle.
For instance if your running 8 tracks, the daw is processing 2 tracks with each core, you may be running fine but near the limit of the processor.
As soon as you add 1 more plugin or one more track, you're tipping one thread into overload.
I'm no expert and assume that every daw has accounted and programed for this, but it could be possible based on the track layout, that two,
more intensive tracks are being processed by the same thread while there is still headroom left on the others. (Completely speculation)
Check your task manager's cpu meter, I found that in ableton I started getting audio artifacts when my reading was past 60-80%.
I found good way to spread the load out between the threads is to send one track to another and split the effects between the two tracks.
In ableton you have the option to Freeze the track which basically renders it to audio and disables all the plugins. You can unfreeze the track when you want to edit the processing.
I had my Q6600 OC'd to 3.6 and it gave me quite a bit of performance increase over the 2.4 at the expense of quite a bit of fan & pump noise.
I eventually got an XPS15 I7 2.0 GHZ and it outperformed my OC'd Q6600 in the same sessions using only ASIO4All thru the onboard realtek audio.
Scratched my head a bit and speculated it was due to the efficiency of the I7 as well as the Hyper Threading (q6600 Oc 3.6ghz x 4 cores = 14.4 GHZ of processing power)
(2635qm 2.0ghz x 8cores(4xPhysical 4xVirtual) = 16 GHZ + Turbo Boost) I know it's a poor way to measure it but in theory it's accurate.
I must say though either it's complete rubbish or I have run into some sort of bottle neck, cause if I use the same math on my current desktop configuration
(4.7ghz x 12 (6 physical + 6 Virtual) = 56.4GHZ) I definitely notice an increase in performance, but it doesn't feel like the 4x that the numbers would imply. I start getting audio hiccups at around 20% cpu.
A simple check would be to get DPC Latency checker and see if there's any discrepancies there.
If you get any peaks, then something is causing latency spikes and you need to systematically search for the culprit.
Could be a display driver, or a wifi adapter, or anti virus software or a many number of things. DPC Latency Checker
Hopefully some of this information is helpful lol..