Ken's description of leaving the throttle on past the top of a loop is spot on. It does sound though that your thrust line is well off - you need a plane which flies straight and level a 1/4 throttle as well as full throttle. If you trim the plane to fly level at full throttle and throttling back then causes the plane to dive markedly, you need more down thrust. This is really the only way to correct the problem, mixing it out on the Tx will only cure the problem half of the time.

I would 100% recommend you sort this out, then when you can use the throttle properly to adjust for wind speed etc. as well and you won;t be fighting the plane the whole time.

James