O2 sensor question

Hello, my truck is a 91 sierra 2500 over 8600 gvw (8 lug) 5.7 4l80E.
I have tried to search and google the info without much luck.
After my truck failed a Calif smog test I set about testing the different sensors that are attributed to the reason it failed (a few points high on HC's).
All the smog equipment and sensors tested ok except for the 02, (I think).
I tested the one in the truck when it failed smog, it came out at 400 milivolts key on engine off as my Haynes manual and the Autozone website said it should, after running the truck for 10 minutes and running it at around 1200 RPM's for 2 min to get the O2 cycling it doesn't, best I have gotten was a slow rise of about 0.01 milivolt rise per second to about 0.590 MV where it seemed to level off, the problem is 2 seperate new sensors do the exact same thing, none cycle rapidly between 100-900 MV as the Haynes manual and the Autozone website say they should.
The 02 is pre-cat stock location, no changes to the exhaust since my last smog check.
I don't even know if this is the reason I failed smog, it's just the only thing that didn't pass my testing. the truck does seem to be running a bit rich (per my ultra sensitive tailpipe sniff test).
I did do all the normal repairs after I failed the smog test, tune up, air filter, clean the TB run a can of cleaner through the gas tank, ETC, then I failed the re-test by 3 points worse.
The truck does not run bad or blow a bunch of smoke, the truckis not thowing any codes.
If anybody could throw some light on my problem I would really appreciate it.

Wow Calif is really tough, this is unburned fuel that you are getting as you know, make sure the fuel evap cannister is hooked up and not filled, it is pretty old, fuel contaminated oil can cause this also, Rememeber the o2 sensor is only responding to what it sees in the exh. stream, so either too rich or lean will skew the switching up and down. Also check the coolant temp sensor, I have seen these feed back cold temps to the ecu and keep the engine rich, check this on a scanner. Here in NYS over 8600 is exempt.

Thanks for your input maxwedge, you have been quite helpfull to me in the past and I really appreciate it.
Yea the smog laws here in Cali are tough, my truck was exempt from many of the tougher standards until last year when some new laws the old governator signed came online.
I will check the things you mentioned, though the evap cannister is connected I dismised the thought that it might be full, foolish I know.
The temp sensor did pass the "installed test", I will be pulling it for the proper test.
On this same note I do have one conflict to check out further, after a lot of over heating and cracking plastic radiator tanks I had an old school brass 4 row Chevelle radiator cut down a bit for a perfict fit in my truck, every since then my temp gauge never shows the truck comming to operating temp, though I didn't like this I let it be because I was just so damn happy to have solved my overheating problem. I have the proper thermostat "190 deg" and all but never checked any further.
I considered this for my failed smog test but figured that the 02 sensor would see the rich condition from the temp sensor and compensate, but in the past couple of days I have been thinking that the temp sensor may over ride the 02 sensor as far as the PCM goes, since I'm not sure of the PCM programing it's my best guess.
The 02 sensor see'ing a rich condition shows in the 400 milivolt reading, "I'm thinking" since the "initial cold ignition on engine not running" reading is 400 milivolts as well and the higher 900 milivolt reading must go to a lean condition.
Since the temp is a known conflict this is where I will start checking deeper, I did "boil check" the thermostat when I had the overheating problem but figure I will start there since thats where I'll start in checking the temp sensor as well.
Thank you again for your input.