Tomposs

I have a 56 Yamaha ybr 125 not a custom, The bike runs fine until all of a sudden it dies, it feels as though fuel is being starved as when it happens it feels as though the back brake is being tapped a few times then it conks out and if I twist the throttle nothing happens, power is still on though ie indicators and neutral light. It then will not start what so ever until I worked out that if I removed the fuel cap it fires up again. I believe it could be pressure in the tank, I have am hoping to look at it myself before forking out monies to a garage. I have put a new battery on since this has happened, also cleaned the air filter and emptied the crankcase breather. Any help would be great!

I have a ybr 125 06 model, it runs fine and starts great when cold... as soon as the temperature outside goes above 15 degrees, it will run, then seem as if the bike is not getting fuel and cut out, in cold weather its fine, since the suns come out I cant ride it for more than 5 minutes without it "overheating" and cutting out mid ride, its now causing me to not get to work and they think im at home having a beer in the hot weather! I have considered smashing it up as pushing it 6 miles in the sun is not great fun. Once I park it in the shade for 5 minutes it starts again, then 3 minutes later stops.