Recreational & Family - Pedal Trailer Advice

I own an 8 year old Alley Cat 1 speed pedal trailer attachment. I am having increasing problems with flex at the seat post attachment point plus flex onto my hybrid bike. Since this is an older trailer, have the newer ones gotten better ? Is there less flex affecting the adult rider ? I am not sure if this problem has developed due to stress/fatigue at the clamp onto my seat post of that as my 6 year old son gets bigger his pedaling effort is affecting my riding. Has anyone else out there encountered this problem ? I would appreciate feedback if the newer pedal trailers (even with 5 speeds) are better in the attachment and side to side sturdiness. Thanks in advance !

The problem is a worn out hitch. I deal with a fleet of trailercycles, for rent, and every year I replace the hitches due to wear. Similar to how a chain wear out by the plates, pins, and rollers rubbing together, your hitch wears from the parts of the universal joint rubbing together. Check with an LBS to see if a replacment hitch is available. I wouldn't worry about your seatpost, it is more then strong enough to handle the stress.

Thank you very much for your reply. I am not concerned at all about the seatpost but I think that you have answered my question. Maybe there is so much metal fatigue where the short quick release skewer holds the trailer at the clamp that it is causing the flex that I feel. Thanks again !