I had similar scores when I started to prepare for GMAT.
GMATPrep : 560, 580, retake : 550 , 620.Kaplan
: 530, 520
Princeton: 520, 570
I would suggest the same as what I did: (It may not be the best solution but it has helped me a lot)
1. Instead of rushing and taking more and more tests. Try to go slowly building solid foundation.
2. Start with Manhattan SC and maintain error log
. (Do SC for at least 4-5 days) Solve as many problems as you can.
After 5 days review your mistakes from error log
and mark the repeated one's with RED (weakness).
3. Powerscore CR
bible : try to read each and every details. Solve as many problems as you can after that for 4-5 days.
4. Do the same for Maths: Instead of rushing to difficult questions try to solve simple questions to build foundation.
5. For RC : I need to work on it . So no suggestions. Practice is the key.
After this try to take one test to check your progress.
I repeat: don't just rush to take test.
GMAT Club Error Log V3.0 - with Timer
GMAT Club Error Log
Spoilt Error Log V0.0.9 - with Timer
Awesome Error Log - By Spoilt and Trianglock