1992 Chevrolet Silverado power steering

i replaced the power steering pump a few weeks ago and put in new high and low pressure lines for it, every thing was fine the first few times i drove it but then the power steering started cutting out at low rpms but it was fine a high rpms, i thought this might b because i had not bled the lines properly so i bled the lines (or tried to), but i am still getting this problem, what could possibly be wrong and what can a do to fix it? any thing will help ive got to have it running before school in 2 weeks so thanks!