Hi Shocky, thanks for your very helpful reply, now that's service!
I wasn't sure what Quadrant is as there's quite a few listings in the Play Store so I've downloaded 3dmark...
Under Ice Storm my result is 10,984 and it says my device appears to be working normally but there was most definitely choppy parts where it looked like it was struggling?? Maybe I'm just asking too much?
And under Ice Storm Extreme (which again is undoubtedly annoyingly jerky)... my score is 6602.
I don't really play very many games but the few that I have done, it's mainly Carmageddon which showed definite lag and far from smooth and Cubed Rally Redline (which is free and caused major lag, you'll laugh when you see the graphics! lol). But yeah in both cases, I'm getting lots of short pauses/lags every 30 seconds or so. That said, when I'm scrolling up and down through webpages I sometimes get it then, too. Also, now that I think about it, I also get the same lag every now and then when playing back DVD quality (Standard definition) Mpeg2 files from the memory card (back in the days when my memory card actually worked) with MX Player.
I'm wondering whether it's all SD card fault related and the overheating is definitely cause for concern which I'm pretty sure is SD card related.
As far as the new Samsung SD card goes... I'm just gonna have to send it back and hope it was a faulty one, I guess? But as you say, given my problems, I suspect the phone is the cause.
I bought the phone from ASDA Direct and the SD card from a reputable dealer on Amazon with good reviews on the card.
The last 3 letters of the baseband version say "MDM" and I don't recall having any OTA updates to the phone since I purchased it a few months ago.
Is everything looking normal? Once again, thank you for your time, I really appreciate it