I've been having these problems for weeks now and I haven't been able to find a decent fix for it.
If I make a complex track on Reason, and Reason alone and my DSP is not high, and it's overloading my processor and then take this track and open it rewired into Ableton, my processor usage is usually around 80% and when I hit play my soundcard is simply not able to handle all the information.
I'm currently using a Roland Cakewalk UA 25 EX external USB soundcard.
I though the sound card could be causing problems, so I lowered the resolution 48 and raised my latency to the maximum so the system would stay stable.
It just didn't, it doesn't ever get stable anymore.
If I do manage to get reason's consumption under control as soon as I start making new tracks on Ableton my processing goes beyond 100%.
I have never had this kind of problem in this magnitude. And I used to route all tracks from reason to separated tracks into Ableton, making Ableton my Mixer for Reason.
It worked like a charm, I managed to drop various VSTs on the tracks and re-do all the mixing and mastering via Ableton.
Now it's simply impossible. I noticed this is specially true when I use the True Bass Refill - which seems to grab a lot of RAM.
My main problem is: I'm not using more effects than I used to. Even older track projects are running like a piece of dodo. Even when I made all my production on Reason, after everything was done, I would route it into Ableton to master it (that 'The Glue Compressor' is simply my favorite compressor ever) but now that's impossible!
Anyways, the only solution I can think of is reformatting my PC. But I REALLY didn't want to do that, I currently have no space for a backup and I don't want to spend money on a new hard drive.
EDIT: BTW: I'm running a Athlon 64 X2, with 2GB Ram and Windows 7 64b. I know it's a outdated spec, but then again, why would I start having problems just now?
Ableton+Reason never seem to use more than 800mb, at least the task manager doesn't indicate anything more than that.
EDIT2: After checking something around stackoverflow it seems that computers naturally get slower, specially when various people share it.
I'm gonna try reformatting everything this week and then post the results.