U seem to have a similar problem like mdmorrissey. He also has random freezes mostly when reading and turning a page.
If u Hardreset the PB and have 14.2 Firmware Installed the pocketbook should be rockstable.
I read about 4000 Pages now, without any !! freeze. I had one freeze with 15.2 but its still beta. Under 14.2 flawless.
What is FBReader 1.81 ? I use 0.90 from
here
BTW do you use your with PB a sd card ? Mine is still without, maybe thats a issue.
If anything is tested and still it freezes than ur PB is maybe broke and need really service.