The usual: Trash Pr Prefs (hold option/alt while launching). Reinstall app if all else fails. 5.5 doesn't have an inherent problem with P2 on my system.
However, I have had problems with P2 files that were corrupted when copying from the card to an external drive. They seem to be sensitive to that, especially with a simple Finder copy where the CONTENTS folder is dragged from the card to a disk. Best practice is to use a utility that does a verification, to ensure a byte-for-byte copy is made.
And, I've also had a situation where FCP7 could import P2 where Pr couldn't. In that case, I just transcoded via Log & Transfer to Quicktime, and brought them into Pr.
I have a simular issue where I cant edit P2 AVC-Intra 1080P @ 100mbs - if I have large/long files it hangs up memory and crashes while scrolling in timeline. I have duplicated the problem on two different editing systems. CS5 woks with the same files just fine -- while CS5.5 just crashes. Tech support has been painfully slow and mostly useless. Waiting for CS6 to fix it.