How do I Fix these Printing Issues?  

How do I Fix these Printing Issues?

I have an assembled Mk3 that works pretty well for smaller prints, but I am having some issues when I do larger prints.
Here are a couple pictures of a recent print.

On the front: the lines should be straight up vertical, but as you can see, they are quite wavy

Is this a problem with my X-axis belt? Is it too loose or too tight? How do I know when it is the right tension?
Is there anything else it could be??

Rotating 90 degrees, there are several horizontal lines along the Y-axis.

I'm not talking about the layer lines, those are OK. Its the gaps and uneven extrusion. I can't say for sure, but it seems to be limited to a range on the Z-axis that these gaps occur - on this print about about 1/3 of the way up, I'm not seeing those lines.

Any ideas what I could do to fix these?

If it helps, this only seems to be on larger prints. Smaller prints (low on the Z-Axis, X/Y closer to centre) seems to work just fine (other than some stringiness, but I working on that with different filaments and temperature adjustments).

Thanks for any ideas!