NEFF U1721W2GB?

My oven fan sometimes does not start up, causing the fuse box to trip out , at first I used to give it a little prod to start it ,and all was fine ... Last week I took it off and discovered that it was all gummed up , ,presumably, ,causing it to stick I gave it a good clean and It has worked for only 3 days and now it is not turning on again! The last time I detached the fan wire ( double spade connections) the element lit ,but after only 3 mins it tripped out again, I would have thought it would have worked o.k. without a fan! Could I please have your thoughts on this ,... Many Thanks Peter Dolby