We had the same problem with 7750s, so now when we upgrade them to 4.0 (or get a unit back from a user complaining about the problem), we remove Help, BrickBreaker, Password Keeper, and all of the additional languages to free up more memory.

This solved the problem for 95% of our users.

lostinspace's tip worked on the remaining 5% that had still had problems. They all had huge calendars, so we set their calendar from "Forever" to "60 days", rebooted, and they've been fine since. (knock on wood)

IMO, running 4.0 on an older device (anything with less than 32MB of ram) is possible, ... just not really optimal. Having upgraded our companies 100+ 7750s to 4.0, if I had to make the decision to do it again, I'm not sure I would. I'd probaby hold those guys at the OS that their BlackBerrys were on until they were eligible for a free hardware upgrade from Verizon.