If you need or want to test RAM…
Apple Hardware Test can check installed RAM, see if it is OK with the new memory…
Using Apple Hardware Test - Apple Support
Rember also tests RAM integrity…
http://www.kelleycomputing.net/rember/
If the OS is booting & stable there is not much to do to make it 'accept the RAM'.
So far we know you think you have RAM issues but all is all we know. It's already been explained that the OS tries to cache data in RAM - it removes data from the cache when other apps require it, there are other indicators besides RAM.
How much disk space do you have free? Too little free effects performance.
Are the logs spewing anything when you have the slowdown? /Applications/Utilities/Console may show issues within 'All messages' or in other log files (it's hard to say which log to look at - focus on the apps that are causing 'wait cursor' or slowness). Copy a handful of log messages around problem times & compare to other times when issues occur. You can post a few lines here, but try not to swamp the forum with many log entries, too much info doesn't help sadly.
There should be logs that indicate when a process crashes or panics - start noting date & times when it restarts on it's own, then look for logs from that time.
Other apps may be using system resources, are there many items set to open on login? Do you have any items installed as kernel extensions & background jobs? How about USB or other devices? Do these have third party drivers etc - are they up to date?
EtreCheck is a simple way to present those details here or use it to get an idea of what is running & installed on the Mac & then check those installed items are up to date & compatible with 10.10.
http://etresoft.com/etrecheck
http://roaringapps.com/ will help show what apps other users have found to work on various OS X versions.
It also helps if you can test doing the same tasks another user account - do the issues persist?
See if you can work inside a safe boot - do the same issues persist. I doubt Ableton will run in safe mode, but you can try - safe mode disables a lot of third party & Apple extensions so it is normal to have poor graphics performance & 'laggyness'.
OS X: What is Safe Boot, Safe Mode? - Apple Support