18 grain pellets are way too heavy for most springers to be efficient and I think they may be causing the gun to diesel which can be seriously upsetting the results.
I would also suggest getting a new battery for the chrono (they can give some odd readings when getting low, especially in cold weather, chrono's like to be at room temp and so do batteries, but using an F1 indoors you will have to be careful of the lighting, energy saving bulbs and flourescent tubes are definitely not good. LED or incandescent bulbs such as halogen work if you can't use daylight.)
Lighter pellets such as RWS Hobby, Falcon Accuracy Plus (JSB Jumbo Exact RS) are often the most efficient with this type of gun, if your gun is sub12 with these then it is probably legal with all other pellets, but don't take that as gospel, you have to try a good few to be certain. Every gun is different and that can mean it might be most powerful with a different pellet, possibly Webley Mosquito (JSB Jumbo Exact Express).