If you see cracks in the boots, they may or may not penetrate into the inner layer of the boot material. Generally this stuff is pretty hardy, but they can get bad enough to let extra air in. A temporary solution would be to get some RTV (gasket maker goo) from any auto parts or hardware store, spread it all over the crack, and then run the motor while it is still wet to suck the material into the crack. This won't last forever, but if this is truly your problem it will get you home, and probably last long enough for your new boots to arrive in the mail. A better solution is to replace the boots.