cottage water system problem

We're having a problem with our lake water system. Please let me know if you have any insight.
System Description: 3 wire submersible pump, about 10 feet under lake surface plus 15 feet height (200 ft run) to pressure tank, switch, valves...
For winter operations (in Bancroft, Ontario Canada), when the pump shutoff is tripped, a valve opens up allowing the water to drain from the line back down to the lake. This means we don't have to heat the line all the way down to the lake; just the short run where the line enters the lake surface.
Problem Description: We purchased the water system with the cottage last fall. Everything worked fine until...
On New Years eve we arrived (at the frozen lake) and I turned on the line heater at the shore for 2 hours before turning on the pump. I left the basement once I saw the pressure in the tank begin to climb. The next morning I noticed a hole in the ice over top of the pump location. It seems the pump had been running all night and melted the hole (from 10 feet under water; 8 inches under ice).
Basic troubleshooting showed that the pump could never again bring the tank up to the pressure (50 psi) required to trip shutoff. It could only reach 42 psi.
A month later I returned and again tried to pressurize the tank. This time I couldn't get the system to reach over 22 psi.
This weekend (well thawed), water barely dribbles into the tank.
More Information: Early in the winter, in an attempt to save energy, I turned off the water line heater at the shore. When arriving at New Years, I turned on the line heater and let it run for 2 hours before flipping the pump switch.
My initial theory has been that, in allowing the line to freeze at the shore, the line had split. The theory goes that as pressure increases, the split opens up and releases pressure such that tank pressure never grows high enough to trip the shutoff.
A couple of observations seem to dispute this theory. First, pressure has kept dropping lower and lower. After New Years I left the shore line heater on, thinking this would prevent further line splitting. Second, this weekend I was able to examine some of the line at the shore and could find no water squirting around (though the line is now under a foot of cold water).
Finally, last fall when everything was working fine, there was a vigorous disturbance of water over the pump when it was running. This spring, the disturbance seems to be less vigorous.
I'm now wondering if that first night during New Years, there was frozen blockage in the line that kept the tank from reaching 50 psi and causing the pump to run all night. This all night running damaged the pump such that it can no longer acheive high pressures.
I'm left thinking I may simply have a failing pump. Worse would be that I have both a line and pump problem.
Does this sound like a failing pump and/or line problem? Any thoughts or recommendations are very much appreciated.
Also, I'm hoping to DIY my way out of this. Is replacing the pump fairly straight forward?
Thanks.
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Just a thought; Is it possible that the pump intake is clogged? It seems to me that in addition to what you have theorized, a blockage at the intake might cause the pump to be unable to supply enough to trip the pressure switch.
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Good thought. I'll check when we go back. Thanks.
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Problem fixed.
Pulled the pump up and found that the problem was a corroded coupler. The coupler joins the water line to the check valve.
Replaced this with a new couple, and Shazam!
Thanks for taking the time to reply.
Mike
John Grabowski wrote:

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