Check your fuel pressure. Try running it without the pre-cat O2 sensor plugged in for a bit.... How old are your tuneup parts? Coil?

no clue how old the coil is. it was on it when i got it.

is the clunking pre det?

i am running w/o the pre cat 02.

how do you check fuel pressure?

just tried replacing all spark plugs to no avail

Need a guage to check fuel pressure. You can get 'em at most any parts store. Check your fuel rail first though, should be a schrader valve on the drivers side rail, (likes like a valve stem for a tire.) Some trucks don't have them though.....

Not sure what the clunking might be..... Have you checked your TPS? Wonder if it isn't telling the PCM that you are indeed stepping on the gas....

ive already replaced it once... i have a fuel gauge but the haynes manual said you need some sort of special adapter. if i let the truck sit for 30 min or so its stops acting up for a while

Adapter is only required if you do NOT have the schrader valve on the fuel rail. Have a look, see if its there.

Have you ever replaced your fuel pump/filter/thatthinginthegastank?

no but i can hear the fuel pump going. i dont see a shcrader valve.

i dont undertsand why its fine for about 10 mins or so after sitting for an hr

thank the good LORD!! it stopped doing it!! ... now back to the 408!

If it comes back, consider the possibility that some chunks from your cat may now be lodged in your muffler. That happens every now and then.

Hoping it doesn't happen, I'm glad you got it fixed!

Low fuel demand. I suspect, but, cannot point and say with any certainty "This is it.".... the fuel filter (in the tank....) is getting plugged up. Either simply due to age, or, something else floating in the tank, so, when you first start up, enough gas has soaked thru to run the truck for a bit, but, when you start putting some demand on the motor, it just can't pull enough fuel to supply the demand. Logically, it works, only way to test though, would be with a pressure gauge.

Without the schrader valve, you will need a short piece of hose, with the valve on it, with the right connections on the ends to put it between the fuel line, and the fuel rail. You should be able to pick up most of that stuff at the parts store.

wonder if it has anything to do with the fact that i usually dont let it get below 1/2 tank and this week she was close to E?

got a check engine light 3 weeks ago. it was cylinder 5 misfire. cleared the code and replaced plug 5 and no more cel.

got a cel 1 week ago with no apparent issues. it began to vibrate a bit when under load while in OD today, and then got a little worse. nothing like it was when i first started this post. if ur upwards of 1800 rpm in OD no vibration, but if you load it in OD at like 1400 RPM it vibrates pretty good. it also sounded odd and vibrates from a stop to about 5mph CEL was cylinder 2 and multiple misfire. replaced all plugs. CEL came back an hour later as it was still vibrating a bit but it sounds normal and doesnt vibrate from a stop very much.

any ideas?

EDIT: there was also a weird fluid, almost like combustion blow by in the spark plug recess. it was not on the threads or electrode but it was on the exposed part of the plug on cylinders 1 and 3.

After reading this I am leaning to a fuel issue. I don't know what every one else thinks but with all of these misfires I would be curious to know what the fuel pressure is.