I know what ya mean. I changed my 40gb HDD to 500gb HDD a year ago. In my ps3 had seagate Hard drive. I installed the hard drive myself. Easy to do if you know what you are doing. Lagging might be your internet connection as well. Because when playing online it does tends to lag once in awhile because we all are on sony's servers.
PS. Just remember all PS3 consoles have small internal laptop hard drives.
I am having the same issues you have had and now I am hoping a new hdd will do the trick.
Did you use the backup utility before removing the problem drive? I am wondering if the corrupt data could be in there and if it would then transfer to the new drive after installing?