Hi,
I had big problems when I started flying could only last about 30 mins then instructor would have to take the controls, at one point I was gonna quit.
Now I only have 20 hrs towards my PPL but I have tried eating lots before going up and have also tried going up with an empty stomach. For me I have a big meal about 2 hrs before then 45 mins before I eat a flapjack. Touch wood the last 8 hrs have been fine. One more thing that is a must, dont leave it more than 2 weeks between lessons, the more you fly the better it gets didn't believe it myself but its working for me.
J