It is possible that your paste cured some, though ceramic pastes usually only take 12-25 hours to cure (at least based on how long AS Ceramique takes to cure). You may be experiencing changes in ambient as well.
That being said, 43C is a pretty damn good temp.
Okay, based on your previous posts, if you were using the 14x cpu multi and you've scaled down to 3.5 ghz, then your current NB speed should be 2.5 ghz. Not bad!
Okay, here's what you need to do.
You need to use the "isolate and consolidate" system that Zebo devised back in the old A64/X2 days. What you're going to do is try to slow down every part of your system but one thing (CPU, RAM, NB) and see how fast you can get that one thing to run. Generally speaking, the first thing to test is the cores, and you can isolate that by lowering your NB multiplier some (try 7x) and your RAM multiplier (not sure how the multipliers are listed in your BIOS, but the lowest multiplier I could use on my system was 1:2.66 . . . 1:2 crashes no matter what). Set your CPU to its highest multiplier (in your case, 14).
Then keep pushing htt speed higher and higher until you can no longer get your CPU stable. Set your vcore to whatever speed you were using to reach 3.6 ghz (you are setting vcore manually, no?). Or you can use something like 1.45v to 1.5v or whatever keeps your temps at 50c or lower.
I use prime95 blend for CPU stability (and NB/RAM stability, see below).
After that, it's time to isolate your maximum NB speed. Run your CPU at a low multiplier (10x is a good one) and your NB at its maximum multiplier (10x). Keep your RAM slow (and you may want to loosen timings on it . . . 10-10-10-28 2T is good). Set CPU-NB and NB to 1.3v (you can go higher, but that's about as high as you should go really) to make sure that voltage isn't a problem, and just to be on the safe side, make sure to run a bit of extra voltage through your CPU (I used 1.37v for NB/RAM isolation). If your chip is anything like mine, you may be able to run your NB as high as 2.9 ghz (10x, 290 HTT), but there are no guarantees. Then increase HTT speed . . . now, I don't know how you are testing stability for your cores, but I used Prime95 Blend and it seemed to do a good job of stressing everything on this chip. Unstable cores would cause a bluescreen and a reboot, while unstable RAM/NB would cause Prime95 to error on one or more threads without a bluescreen. Anyway, try to find the maximum NB speed at which your system will still boot, and then see if you can run Prime95 Blend. Just do 5 minute tests until something errors, then back HTT off a bit and try to get it to run for 24 hours straight.
We'll worry about RAM later.
If you haven't been setting your voltages manually, now would be a good time to mention that.