Swan 250's, and even the 250C's, are well known for their drifty oscillators. Set the receiver at one location and signals will just drift on by.
Ah, yes.....the good ol' Swan TooDrifty. Just wait a few minutes, it might drift back by. Even in a time when some drift was still expected and tolerated, this thing was pretty marginal.
As to the Seneca, WA2PJP who is a member here has one that he rebuilt from the ground up, addressing any issues along the way. He'd be a good one to talk with for anyone considering setting one up. Not a bad set from the little I've seen. Definitely better than a Sixer!