You don't give too much Lee.
It does sound very much though as if the throttle linkage/cable might be sticking. If it isn't working freely that will jam the throttle open or shut to various degrees regardless of your foot position. Try testing that the linkage or cable is working freely. Grease all the links anyway as it won't hurt and maybe run lubricating oil down the cable if it has one. You may just need a new cable or links - easy to get new or second hand ones.
At the risk of stating the obvious, have you serviced the car lately? The problem could be due simply to a service part or setting.