Texgun,
Sound like you may need to do a good cleaning (disassembly of the receiver). Many times when a handiman tears into an m99 to fix a feed problem they end up with a rotary magazine that won't feed properly. Make certain you find a good gunsmith who knows the m99 and its secrets! The tension on the spool spring is critical.
The other possibility is that if you are using handloads, the OAL is critical for proper feeding and might be too long for the magazine.
Jim