exhaust help

Hi to all!

i would like to ask if someone after fitting the exhaust mine its a ktr with 3 collars connected (4 with the decat) facing the following problem:

when the car is spinning the first gear and the wheels have some grip in
a small piece of road and after that they loose again the grip i am hearing all the exhaust to move and thats why the engine and the gearbox mounts have a normal flexibility.i checked all the collars and are all very very good and tight.i am quite sure its from the ex. cause this prob i face it when i fitted in.
does anyone has the same prob ever faced and solve it?
plzz need help guys

nikos