It can't be a big leak if the engine can suck in the contents of the overflow bottle. Large leaks will usually prevent the system from generating any vacuum to suck the bottle down. Since it's erratic, and since you've already had some hose issues, the first thing I'd do is feel the upper and lower hoses when it overheats. The lower should be hot but 20~30F cooler than the upper hose. If the upper is hot but the lower is relatively cold, then the lower hose may be delaminated internally and collapsing from water pump suction. If they're both relatively cold then you have a low coolant or blockage situation. Since it's losing some, first I'd see if the coolant is disappearing outside or inside. Try some UV dye. If there's no blockage and the magic glasses don't reveal anything, then there's a good chance it's a head gasket.