crf 450 R when does it need a new top end

I was riding my crf 450 the other day, full Throttle 5th gear in sand, the bike back fired a couple of times and stopped running.  So I stopped, kicked it a couple of time it wouldn't start, so I pulled the choke and it started right up and I rode home like nothing ever happened.  My buddies said it might need a valve job or top end.  Does this sound firmiluar to anyone?  If it does need a new top end, about how much should it cost?  

you need a new top end if you cant get your valves back in spec.if your doing the valves do the piston also,if you do it depending on the parts you buy $500.00?a shop is gonna be at least $850.00 with stock parts(oem).try shimming first.  
---------------- different toilet,same ***

Thanks, maybe I'll just take the cr 250 R to the desert this time.  

I wouldn't base needing a valve job and rebuild because of backfire and stall.

I would check the valve clearance and also check for a air leak or exhaust leak first.  
---------------------------

I love this dirt stuff.

Thanks, I have about 12 days before Baja trip to figure it out.