CPU usage @5% during 2 minutes

Dear php list! I have a real big and strange problem I ran into few weeks ago. One of my clients complained, that the intranet software began to run very slow, almost unusable. Since then I have almost tried everything to get the thing work. The facts: On P3 architecture systems (I have tried 3, including the server at the clients office) runs very slow and with about 0-5% of CPU usage. The code needs about 2 minutes to get completed. On a P4 it is ready in 2 seconds with 100% CPU usage. I have placed a debug function into the code. It is a string that collects unix microtime each time within the loop. This loop has approx. 10k cycles. The loop does nothing than echoing a row. Sometimes there is a 4 second gap between two cycles, sometime only a few microseconds. The original system was installed from Debian Woody, but the problem appears on a Sarge system, and when using apache&php compiled by me. No configuration file fine tuning seems to work. The most strange thing is, that the problem appeared at a time, when no changes to the program was made. ANYBODY RAN INTO SOME SIMILAR THING?????