I received an OS MAX-S 35 from a friend a while ago and just decided to try it out. He had no luck getting this engine to run. The engine looks good and has great compression. The problem is that it will run great for the first minute then starts to run rough and eratic. It'll stay running that way for a while till it dies. I've checked the tank (Sullivan round clunk on Uniflow) for debris and there is none. Fuel draw from tank is fine. PowerMaster GMA 10/25 fuel used w/OS #6 plug. APC 10/5 prop. What could be wrong or how can I correct this?

Correction. It's PowerMaster GMA 5/25 fuel.

Ditch the plastic tank. Use a good RSM or Brodak uniflow tank. Not saying plastic tanks are always bad, just sometimes. If you have a muffler installed, use muffler pressure to the uniflow. That's has worked for me with S-35's. Also, if fuel is all castor, dump an ounce of synthetic or "Aero Oil" into it. Helps keep the castor build-up down. Another fuel thought: Mine run good on 10% nitro, better on 15% nitro. Have not tried 5%. May want to give 10% a try. 

The OS glowplugs have changed from "shorts & longs" to a new metric length, which is a "medium" length. A glowplug that is recessed from the combustion chamber doesn't usually work very well at all. Try a Thunderbolt, Zinger, K&B, or SIG R/C Long. I do not like Fox glowplugs, tho I suppose somebody must.   Steve

That is totally CORRECT!

#1 Reason: Perhaps due to the angle/shape design of the intake and exhaust ports,some leftover burned spray may accumulate on the glow plug element chamber and cause the filament to cool off. This will cause erratic runs and engine flame outs specially on outside maneuvers or upside down engines. The fuel/oil tends to settle down on a hemi head also. Even worse when there is back pressure on the exhaust. I had an engine that did exactly the same response. Took the head off and noticed that I had combustion tanning all around the plug but not close to it. Analyzed that had good heat but not in the center. Put an R/C plug and still the same result. I tried a FOX 4C and it worked great!!!  

Measured the 2 glow plugs and they had the same depth. Measured the FOX and it was taller. The 2 Plugs were below the chamber and the Fox was above. Many engines do not like all 100% Castor. It is a little thicker to exit the engine. I use syn with 20% premixed Castor and it is perfect for any engine. Klotz Racing TechniPlate with 20% Benol. 10/18 mix on Methanol, 1/2oz Acetone/ga and 2 squirts of Boca Bearing Aero oil.
I used to buy my own and it was always a different "feel". 50 some odd years buying fuel and fighting with engine runs. Now, it is always the same cheap and fresh with high quality chemicals.