LSAT for Critical Reasoning

I recommend that you read PowerScore LSAT CR Bible !

I've read the GMAT Powerscore CR Bible. It certainly helped, but I find that only 20% of the book is useful.
Should I continue with the LSAT tests?

If you have time, doing atleast 2 or 3 LSAT CRs per day, wouldnt hurt

I've gone through the powerscore logical reasoning bible. I would say that if you've already gone through the critical reasoning bible that is all you need. You would be better served just going through the critical reasoning bible again. One thing that I do that helped improve my confidence in critical reasoning is I use lsat test. The lsat logical reasoning, imo, is the equivalent of 700 level questions on the gmat, so to practice with the toughest CR questions use actual past lsats. What may be happening to you is that you're using different prep companies CR questions and their logic isn't exactly what it should be for that section. I believe it is more valuable to use questions from professional test writers than it is to use questions from prep companies. Buy one of the books that has 10 past lsats and use it for logical reasoning and reading comprehension when you run out of material in the Official guide. One word of caution though, they only give answers. They don't give explanations. There is an lsat book that has past lsat test with explanations but they only provide 3 test in that book, so I prefer to get the 10 test and figure the answers out on your own. That way you get the best bang for your buck.