There seem to be two obvious possibilities. One is that the engine is experiencing an unusual level of vibration. The other is that for one reason or another the thread has become loose, which makes it very difficult to keep bolts tight. Are any other screws and components coming loose?
If the engine is rarely dismantled, I'd suggest putting some loctite on the bolt threads. Don't use one of the exotic high strength versions of the sealant of course - you do want to be able to remove the bolts if necessary. If the machine is dismantled regularly, I'd be inclined to switch to studs and nuts instead of bolts, and anchor the studs in the block with loctite.
