Handle is probably cross-threaded and pooped out. I've gone through two in three years, but I'm an eccentric shifting monster. The rod usually lasts longer than the knob.

Go to any auto parts store that sells Hurst or other shifter knobs or balls (Autozone, Orielly, Advanced Auto, etc).

Find shifter knob you like, tell clerk you want to make sure it fits onto your shifter rod. Go out into parking lot and try it out!! If it screws in, buy it. If not, try another!

Don't drop a dime on that kind of stuff without checking it out first. Most stores have no problem with that method. If it turns out its the rod that has bad threads, just look for the rod at stores or on Ebay, not the entire shifter assembly.