Bring to LBS and let them figure it out, perhaps the seatpost is too small ect.
Replace clamp with one that has a bolt, not a QR
Get a clamp that fits around the seatpost and bolt it shut, keeping the one already on the seat tube. That will keep the seat post from slipping for sure.
If the seatpost is small your LBS may have a shim for it. You might be able to wrap some aluminized tape, used for heating ducts, around the seatpost.
2000 Montague CX, I do not recommend it, but still ride it.
Strida 3, I recommend it for rides < 10mi wo steep hills.
2006 Rowbike 720 Sport, I recommend it as an exercise bike.
1996 Birdy, Recommend.
Wieleder CARiBIKE (folding), decent frame.