First, Do you know how much Sugar is in one of those Slim Fast shakes???? Here is the info go to this page
http://www.slim-fast.com/products/pr...product_id=270
I think honesty its the sugar your taking in. One of those shakes has 34g of sugar. Sugar is one of the last things to break down in your body.
I too think you need more food at lunch. A shake and one of those bars for lunch I don't think is enough you need protein or something. I would eat like a Heathly Choice meal for lunch with some fruit.
I think you feel good in the morning cause your taking in lots of sugar then you bonk at the end of the day. Its like a natural high. Up in the morning down in the afternoon. Do you feel fatigued???
I would really consider making some changes to your diet I know your losing weight but you can still lose weight and eat good. Now that you are cycling you do need more food and your body is in a sense trying to tell you this.
This is only my opinion you can totally ignore what I'm saying if you like!!!