Stiff shift boot -

[Durt]

I realize this has been posted at least once already - so I apologize for the redundancy (yes, I checked the archives,) but it's still a VERY irritating - and possibly dangerous problem for me. Had my SPF for close to 14 years & have never had this issue before. Got the boot from Finish Line about two years ago & it still pops me out of 1st, 3rd & 5th. I've boiled it, put it in a bag with silicone spray, left it in 1st at all times at rest & it's as stiff as the day I bought it. Yes - the hole in the transmission tunnel is centered.
Is the culprit possibly the linkage on the Tremec 5 speed getting tired? Do they get loose over the years? They certainly aren't adjustable to my knowledge.
I checked with Cobra Restorers in Georgia at Lynn Park's recommendation for a new boot - but it looks suspiciously like the one I'm running now. I know a lot of folks recommend a leather boot - but I simply don't want to go there. I'm going to try to run the car without a boot & see if it still pops out - but absent that - does anyone have a line on a boot not made of aircraft tire rubber?

Thanks -