If there is plenty of water in the well it sounds like the jet and foot valve isn't set below the water enough or the well doesn't recover fast enough to keep water above the suction.
You may be using an excessive amount of water due to leaking faucets or stuck or leaking commode.
A flucuating pressure indicates to me that the foot valve was sucking air meaning not enough water or not set deep enough.
The seal at the top of the well is only a sanitary seal and has nothing to do with the pumping system except maybe holding the drop pipe suspended in the well.
Be advised that not all plumbers are professional pump installers! If a professional (certified) pump installer (or installers) dropped the pipe, jet and foot valve in the well it was their responsibility to recover it. An experienced pump installer would have had no problem recovering the drop pipe from a 2 foot diameter concrete cased well.
$900.00 sounds like a lot for just labor, the drop pipe, jet and foot valve. If it included a new quality made pump, that's not too bad.