Those issues would have been noticed if they were bugs, and there's a distinct lack of posts reporting the issue. So it's more likely the theme you're using is buggy or corrupt. Either that or your video card or RAM is dying a slow death.
Mint Testing Team & Mint Donor #3606
KDE 4.14.2, custom preemptive kernel 3.16.5,
Intel i7 4820K @ 4.7GHz, 32GB Corsair Vengeance Pro
3122MHz XMP, 4 Samsung 840 PRO 512GB SSDs in
RAID0, 6TB HW RAID10, 3 x 24" Samsung UlHD,