Clare,
I had the same problems when I started with my Singer 66 treadle. You have to train your feet to move the correct way.
I did that buy turning the hand wheel as I let me feet sit and feel the treadle plate move under them. Then when I started to move my feet they still couldn't get the rhythm so I got mad and continued turning the hand wheel by hand as my feet fought to get the hang of it.
A do or die thing. Suddenly, like when you have an epiphany, my feet and brain connected and off I went. It took a couple days to get it down pat but I did it.
Once you get the rhythm you can actually feel the machine and with a glance start it going the right way.
Now, be sure to clean any and all lint, crud and thread out of the treadle pivot points, the pit man shaft socket and bearings and then oil it good. Put paper towels under it, don't be chinchy with the oil. It's probably been a while since it was oiled good.
Joe