It will most likely be the flasher canister, that's the little relay that causes the lights to go on & off. Any parts store should be able to sell you a new one for a reasonable price. A mates pulsar was doing exactly what you say your's is doing & a new canister fixed the problem. 1200's usually have a 2 pronged canister, as you get them in 2 & 3 pronged varieties. Have a look up under the dashboard & you should be able to see the old one.