GPU is the problem, get a better one.

You mean the PCIe slot? Don't worry it is very durable 

The real cause is the card, either too heavy or too long and causing slight "bending" or "flex", it should be ok because the PSU shroud of this chassis is very close to the GPU which can act as a "support" to the card.
GTX960 2GB price now is around RM 850 (more or less), SLI doesn't provide 100% perfect GPU scaling so you might end up getting extra 50% boost only (this is different for each games). Another issue is the VRAM, 2GB is barely enough for any games now even in 1080p, do remember VRAM will stay the same even in a SLI setup unless the games you play support VRAM stacking in DX12 (none available currently).

GTX1060 3GB is just slightly above RM 900 now, which provides twice the performance of your current GTX960. By going for this path, you don't have to worry about the performance of SLI, or whether the games support SLI or not, the only thing you need to worry is the VRAM.

Higher end single card is always on top of the "buying" priority when you compare it with weaker cards in SLI. Real life example, RX480 on CF vs GTX1080, AMD claimed that RX480 on CF can beat GTX1080, yes this is true, but not entirely true though, it is true if only raw performance is involved and with perfect scaling, but how many games out there have perfect or near perfect scaling? Only few titles, like AC: Unity and Crysis 3 

It is cheaper to get RX480 CF (RM 1300 x 2 = RM 2600) compared to GTX1080 (RM 3000), but the power consumption will be much higher, 300W vs 225W (just a raw peak estimation, not reflecting to real life power draw). I am not going to mention the requirements on mobo and PSU since you already have them.

My suggestion is sell off your GTX960 (probably can get back RM 500+) and get GTX1060 6GB/RX480 8GB.