FX 8120 VERY SLOW ):

Perhaps the PC is locking up (or an App crash)?

From results I've seen an FX-8150 is between an i5-2500K and i7-2600K in terms of performance under Cinema 4D.

I'd expect your 8120 to perform around an i5-2500K.

Does the app show up as "Not-Responding" under Task Manager?

nope, a 2 object video @ 60 fps slightly higher then 1080p and about 3 seconds long took about a minute so rendering a decent animation will take a VERY VERY VERY long time ):

Try to overclock it, since that kind of work will get the zambezi really hot I also suggest an aftermarket cooler, could be the hyper 212 evo from coolermaster, it's cheap and does the job

I just upgraded from a 212+ to a TPC 812.

What settings do you have?

Antialising might be set very high, change it to geometry.

It can be;

1] software caused crash, drivers etc etc

2] or hardware related...ie: the chip is defective....but that will be more general than specific task related.

reinstall the OS
issue solved.

maybe try a little overclocking, turbo core is from my opinion a waste of time just turn it off

i have mine at 4.2ghz (multiplier at 21)and runs very smooth on stock voltage, with a ZALMAN CNPSX10 performa.

something odd is that from 4.2 to 4.6ghz on 1.425v doesnt seem to gain nothing at all, so i left it at stock voltage and 4.2ghz