B737 departure from ONT described multiple altitude assignments; a TCAS TA; all allegedly because the ATC controller erred in separation efforts.

Narrative

On climbout of ONT outbound on the PDZ 130 degree radial on the PRADO 6 Departure; First Officer (Pilot Not Flying) checked in with SOCAL climbing out cleared to 7;000 FT with restriction of 4;000 FT. SOCAL on 134.00 cleared us to climb and maintain 13;000 FT. We read that back. We were in a light B737-500 series climbing approximately 4;000 FPM. Passing through 7;800 FT; SOCAL told us to climb and maintain 7;500 FT. First Officer read back the clearance. I told him we are already through that altitude; tell ATC we can descend back. ATC then cleared us to 8;500 FT before the First Officer could query her on the 7;500 FT altitude. We both verified the altitude in the MCP and I leveled the aircraft off at 8;500 FT. At that time SOCAL told another carrier that he was to look for a Company aircraft at 7;500 FT and he should be level at 8;500 FT. I told the First Officer to query ATC as to our altitude. He told SOCAL to verify our altitude because we were level at 8;500 FT and that is what we read back. She told us to descend and maintain 7;500 FT. I disconnected the autopilot and descended back to 7;500 FT. Upon disconnect; we received a TA on the TCAS. Upon reaching 7;500 FT; the other carrier crossed over at 8;500 FT and no RA was ever received. SOCAL then recleared us up to 12;000 FT; and then her next radio transmission was up to 13;000 FT. She then handed us off to the next frequency. First Officer could have read back instantly that we are already through 7;500 FT upon the clearance and told her we will need to descend back to obtain that altitude. I really think that SOCAL never listened to any readback of ours. She was very busy with several aircraft. 4 different altitudes were given to us in such a brief period of time.