I'm having a problem understanding where I stand with my practice Quantitative scores. If anyone who has taken the GMAT can offer their advise, I would appreciate it.

Here is some information on my quant. preparation:

• Kaplan Math 2009 - I average just under 2 minutes for these problems. Difficult DS problems seem to hurt my time and scoring.
  Kaplan GMAT 2009 - Same as Kaplan Math.
  OGGMAT 11th Ed - The practice problems here seem very easy. I average very few mistakes and about 1 minute per solution.

Here's some information on my practice test scores (in order that I took them):

• GMATPrep adaptive: 38 Quant average
  Powerprep adaptive: 39 Quant average
  Kaplan adaptive (#1 and #2 taken) - from Kaplan 2009 GMAT CD: 36 Quant average

So in essence, I've seen a decline in my Quant scores. In my opinion, the Kaplan pre-tests are much more difficult than GMATPrep or Powerprep. Which score should give me a better idea of a projected actual GMAT score?

In addition, what is the best source for DS practice problems that most accurately reflect GMAT questions?

I appreciate any help you all can give.