Loose crank - Sugino 75

Since I finished the bike about 6/8 weeks ago I seem to be having to tighten up my crank(both sides) atleast once a week. I'm slightly concerned as I've never had to do this before on my previous bikes. I have to tighten over 180dg sometimes.

I have a Sugino 75 on a Sugino 75 bb

Any help much appreciated

I had this problem and eventually resorted to lock-tighting the chain ring bolts. This solved the problem though I presume it would have been a bastard to change your chain ring if needed!