Aftermarket coolant reservoir

[ChadCy75]

I'm having trouble with my '99 almost overheating, and I suspect the main problem is that the threads on the coolant reservoir aren't letting the cap seal properly. So, before I wind up blowing either the Lower intake Manifold or the head gasket, does anyone have ideas for either an aftermarket coolant reservoir, or even one from another car that I could rig in to replace the one that's in there now? I'm sure my problem isn't either of the gaskets, yet, because it doesn't have the symptoms for it. It was running fine until a few days ago, when I blew another reservoir cap, and it seemed to get MUCH worse after I changed the spark plugs yesterday. The electrodes on the old spark plugs looked like the tip of a very sharp pencil. I have only had this car since May, and I'm slowly getting it back into some kind of decent shape, because the previous owner definitely neglected this car. The spark plugs aren't my only evidence, as it took two oil changes to get oil that still looks clean after three days, and the interior will have to be replaced at some point, unless I can't get this over-heating problem worked out. Any ideas you guys have will be greatly appreciated, because after three years of driving a beat to death '89 VW Jetta, this Alero is so sweet, and I don't want to lose it! Also, any ideas you give me will almost certainly help others who start having the same trouble, hopefully before they blow a gasket, because I would almost bet the farm that a lot of the gasket trouble with these cars started with the coolant reservoir tank cap not seating or sealing correctly!