Thanks for the replies.
That's interesting about the weather affecting the 12gram's output pressure... for some reason, I thought it wouldn't have much effect because the volume is so low.
I guess I will try the double spring trick or perhaps visit an auto-parts store.
I took the marker apart... the receiver is stripped for the first 3 threads. The last 4 or 5 threads look fine, though. The plug has an larger knob that prevent it from screwing in all the way. If I had a stock knob, I could probably screw it in farther and catch the intact threads more reliably.
The plastic ball bearing is so worn that it is flat on one side. I guess I'll be shopping for ball bearings and springs this x-mas.