My 2000 nissan xterra idles a little ruff, and lacks power when i take off, its has 105,000 miles, the other day the service engine soon light came on when it was lacking power on that off then went off again, what could be the problem

If it's a V6, it could be, or it could be the bypass house (which is recommended to be replaced when you do the timing belt/water pump at around 105,000 miles)
www.clubxterra.org will be able to provide you with more answers

Also- the rattle is most likely the valve lifters. The lifters on this engine and the model previous (I have owned both to 180k miles or more) lose pressure when the motor is off. They re-pressureize with engine oil after the motor starts and tap a little until they "pump up" to full size. Just don't drive until the rattle dies down and as long as it is under 30 seconds or so- you will be fine there.

On a 3.3L engine V-6, Nissan recommends replacement every 105,000 miles, note that this is an interference engine, that piston to valve damage will happen in the event of a timing belt failure.

Xterra with 2.4L and 4.0L have timing chain only, no timing belt.

60-80,000 miles is a great benchmark for ANY vehicle. nissan recomends 105,000 miles which seems a little excessive to me. be careful because this engine is listed as an interference motor and WILL cause internal engine damage if it was to break.

If it's the indicator that literally says "service engine soon", and not the check-engine light, then you have no worries. That's programmed to come on at set intervals to remind you that routine service may be due soon (oil change, inspection, tire rotation, etc). It's not an indication of failure. Failures will be shown by the check-engine light. That one is nothing to worry about. Your owner's manual should have a procedure to turn it off.

9 times out of 10 it is nothing. My engine light stays on. If it really bothers you then you can take it to any oil change place and there is a sequence that they do.`

Is your "check engine" or "service engine soon" light on? Lacking power is usually an emission system problem, for instance possibly a bad EGR valve or mass air flow senser. O2 sensers are common to fault but normally doesn't cause power loss usually causes alittle rough idle. Could even be a pluged catalic converter to. have the truck plugged into a scanner, that will tell you the trouble codes and fault info.

I had a problem on my Nissan Xterra 2000. Symptons sound similar. In my case, it ran fine for a while and then suddenly the engine starts to stumble and lose output. At one point it did not even have enough HP to move. Irregular idling too. Thought it was water in my fuel. Bought water remover from the store and added to gas tank. Problem did not go away. Instead, "Service Engine Soon" light came on. Had to have it towed to the dealer as there was no way the engine could keep up with traffic. Dealer replaced the "Crank Position Sensor" today. Parts $33.00, Labor: $300 (about 3 hrs worth of work). Seems to work fine now but I will wait and see. Dealer also stated that Distributor bearing appears to be noisy and will need to be replaced sooner or later. I will wait for it to go before I fix that.

The knock sensor is not a huge deal to replace and will not turn on the light. It should be dont though.

The crankshaft sensor is located directly between the engine and transmission. It is dead center at the bottom.

nothing,, that light is just a service light....a reminder that you regular maint is due soon...thats all..ur almost due for ur 30k major service