Ok, thanks for all your replys.
I think I've narroved a problem a bit.
I took out my "spare parts" only Amiga unit out of the closet, installed a fdd which it lacked and tried out all those games I was whining about not working on my second, troubling Amiga. Both units have exactly the same MBOs. Rock Lobster rev 6A.
On my spare Amiga everything works without any problems. So I figured, it must be a hardware problem. I then started replacing all the chips (Denise, Paula, Gary, CIAs, MC68KP8 ... but to no avail. The only IC still not replaced is Fat Agnus since I don't have the necessary IC removal tool to do so.
At this point I'm certain that it's not the dirty fdd heads, not the Amiga Explorer transfer, not the Rom sharer board (tried without it, just KS 1.3), not the viruses (checked the disks with several av utils) that was bugging me, but a hardware issue of some sort after all.
So, what do you think, could it be the Fatso giving me the headaches all this time ?
I'm in process of ordering two things:
1.) A PLCC extraction tool ( such as this:
http://www.jonard.com/jonard-ecommerce/control/product/~product_id=11029 ) which will hopefully allow me to replace that last non soldered IC on the MBO
2.) Another rev 6A MBO in case the removal of Fat Agnus doesn't go smoothly, or it turns out it's not the Agnus but some other hw malfunction which I won't be able to fix.
Anywayz, thank you all for listening. Your help has been greatly appreciated.