Invacare Perfecto 5LPM O2 purity drops when raised past 3 LPM?

Servicing an Invacare Perfect 2 V 5LPM o2 concentrator. Machine came in for repair, checked o2 purity 1-5LPM @ 89% o2. I replaced sieve beds, When machine is running at 3LPM, o2 purity is 95%, when i raise the 3.5LPM-5LPM o2 purity after 10 minutes drops to 70%. I temporarily replaced the compressor & 4-way with a rebuilt donors, just to troubleshoot the problem. Any ideas what could be the problem?