Hi
    Out flying yesterday with the OS 25 VF on pipe, when I pulled out upright from the
reverse wingover the motor almost stopped and on recovery from the overhead eights had the same
thing happen,the rest of the pattern was OK.A few more fights and the issue seemed to go away?
Any ideas.
Thanks   

   Inverted engine? On pull out, G forces really load up stuff on the glow plug, like any debris, oil, excess fuel. You might have had some sort of build up on the plug and and it interrupted ignition. That is why I run R/C type plugs with an idle bar to help keep crap off the plug element. It's worth the time to just examine the plug and see what you can see.
   Type at you later,
  Dan McEntee

Hello
Interesting malady. If the fuel is fresh I would try a new plug, then next step would change the tank as it seems to starve of fuel after being overhead.
Will be interesting to see others replies/ideas.
Regards Gerald

Not piped but that was like the Glow Plug Warning Light coming on for my OS35s I ran back in the 70's.  You have one of those on your handle don't you?

I used shielded plugs too. 

Ken

   That could very well be part of the issue. The element reacts with the methanol in the fuel to keep it glowing after the battery comes off and that may cause some degrading. Who knows what effect today's nitro may have on it. I've seen carbon build up on the idle bar so bad that you couldn't see the element, and the engine finally would not start. This plug was in a ST.60 for several years! And to the flip side, I have seen Fox plugs that lasted only one run, would not glow with a battery after one run!! And verey thing else in between is possible also!
   Type at you later,
  Dan McEntee