The valve keeps pulling out of the tube on my front wheel after about half an hour of riding every time I go out. I've tried various makes of tubes and recently replaced the rim tape after I noticed the original fabric tape had shrunk and was partly obscuring the hole where the valve goes through, which made no difference. There's no damage to the rim
The rim is a Mavic XC717 and has the little plastic insert still in place and the tyre is a Bonty ACX. The back wheel is the same set-up, and is fine.
Help, its driving me crazy and costing a fortune!