Baffled!!

My Macbook Pro mid 2010. Started life on 10.6. And by this year was running El Capitan - until...

The other day left click went away, right click went away. Two finger touch would bring up a menu but no way to select except via keyboard navigation (left arrow, right arrow etc.). I also had Yosemite on a second partition with a third partition clear for a future install of Sierra. I booted into Yosemite and full functionality returned. I booted back to El Capitan and had no mouse or keypad. I booted back to Yosemite and lost keypad and mouse just as with El Capitan...

Ah well - maybe the Mac is at the end of it's life - that's what several 'experts' were telling me.

I put in the original install DVD (remember those). Installed 10.6 on the spare partition and that's what I'm writing on now. So why no keypad and mouse on 10.10 and 10.11? Obviously not a hardware issue as 10.6.8 is working perfectly.

Someone is going to enlighten me on this forum. Please be nice... I've fixed hundreds of Macs for many people - even ones that the guru's said were toast. There must be something fundamental that I'm missing right?