I just finished building an 80'' extra 300S with a DA 50. Y'day when i tried to start it up for the first time, the engine ran well for the first 30 secs. Then all of a sudden the throttle(hitec standard) and both aileron servos( 625 MG) started to jitter like crazy. I thought maybe its interference so i shut it down. When i started it for the 2nd time the same thing kept happening. the 3rd n 4th start bore the same result. On the 5th start i tried to troubleshoot the problem n narrowed it down to 3 issues:
1. a bad servo amongst all three (possibly throttle servo)
2. Loose plug connector
3. Bad servo extensions

Im new to gassers so pls tell me what the possible reasons could be? am i looking in the right direction? what can i do to sort out the problem?

I had the same problem on my extra 330, turned out to be a loose connector in the right wing. Every time I flipped the prop the right aileron would go full up.

THE REAL QUESTION IS ONCE YOU STOPPED THE MOTOR DID THE RADIO GLITCHING ALSO STOP, IF SO THEN YOU ARE MOST LIKELY GETTING INTERFERENCE FROM THE MOTOR AND WILL HAVE TO ISOLATE THE RADIO FROM IT TO STOP THE GLITCHING.

Check all your connections, make sure they are nice and tight. Also make sure there are no metal to metal contacts. When metal vibrates it can cause a RF problem. So if your using a metal clevis on the throttle with a metal control rod, change them to nyrod, attach an unused servo arm to the throttle(make sure its plastic) then you can use a metal clevis, the plastic acts as an insulator to keep the metal from vibrating and possibly causing RF interfernce. Also check you RX, is it at least 18 inches away from the ignition for the engine? The further away you can put it the better it will be. Also are you using a FM rx? If so try using a PCM rx, they are designed to block out RF problems and have a better recieving function. Just be sure you get one that will match your radio, meaning if your using a JR radio you have to use a JR PCM rx, same goes for futaba and hitec etc etc. Hope some of this helps.
MC
PS: I forgot to say make sur your TX is able to or has the PCM function to use a PCM rx.