Popping out of gear

I don't know what the center bearing has to do with your problem. I'd say it has to do with the clutch. I had a similar problem, with it popping out of gear. Then after replacing the shifter bushing, it was gone.

These are the two parts that normally need to be replaced to resolve this issue. I have also seen where the two steel pins in the aluminum housing that the shifter seats into have become loose over time. I had to replace that housing at around 180,000 miles on my 1991 V-6 truck. One of the causes for this failure is the gear shift boot tearing and allowing road grime and dust to enter the housing and transmission from under the truck. Check to make sure your inner boot is not torn while you are in there.