I posted this in another topic under fish health and illness, on swim bladder problems.  Sounds like your fish is having this problem to so please read below.

This is a most common problem with egg shaped fish...it can be a swim bladder problem due to damage or genetics or it can be from constipation.  The swim bladders vent through the intestines and when blocked, the fish either float to the surface or sink to the bottom each time they stop swimming.  It is very important to treat right away or the problem can become permanent  However, once inflicted with this problem, the fish is likely to have problems repeat themselves again.
The diet should be changed to a more digestable one...high in fiber, so chopped red lettuce, peas with skin removed, and spinach are good to include in your fishes diet.  Worms can also act as a laxative and help relieve constipation problems.  Frozen foods for amnivores are also good.  See your pet dealer for these.
Lastly, this problem can also occur because of sudden temperature change or permant damage to the swim bladder and also a bacterial infection.  for the latter a antibiotic called Furanace can be given.  It must be ingested tho to be effective.
My sister had a goldfish with swim bladder issues for years...it lived to be about 10.  every time she cleaned the tank it would develop swim bladder problems again...but she swore by feeding it peas and said it gave it some relief.
Good luck!