RE: Diving on downwind?
throttle controls altitude
elevator controls pitch.
statement made says diving not loss of altitiude.diving to me means a sudden change in pitch not a gradual loss of altitude,therefore this being an older plane that something has changed to create this problem.if changing the glow plug does not work one must look else where,the next place to look is in the elevater control from servo to elevater to see if all is operating properly,could also be rubber bands on the wing breaking causing a problem or not enough.or the wrong size.should be #64 and at least 10 per side.should be changed each flying seasion.break a prop recently?did you replace it with the same pitch prop?usually a worn engine will idle poorly and the top end will be down and this will show up on take off and climb out.