Repair, replace, or????

Hello
I'm new here, and am probably jumping the gun a bit by not having all the information that I might be needing. I have a log home in northern Michigan, and my well is 75 feet deep. The water that comes out is excellent in quality, no smell, no after taste, just the best well water I have ever drank. I don't even have a water softener. The well was put in when the home was put up in 1987, it is a four inch well, it is a submersible pump (I have the bladder tank in the basement) and I was told by the previous owner when I bought it, it was a half horse power pump. I had a geo-thermal system installed about five years ago, with an open loop discharge. When the geo-system was installed, everything seemed to work just fine. The system would run, I could take a shower, and wash clothes all at the same time. just lately, (this past weekend) the wife complained that while the geo-system was on, water pressure out of the shower was barely a trickle. The same out of any faucett in the house. In fact, I have a tankless water heater, and the water flow wasn't enough to keep the tankless heater running. It requires atleast 3/4 gallon per minute to keep the burner on,,,, wouldn't do it. So, I went down stairs and looked at my pressure gauge. While the geo-system was running, the pressure would drop to 12-14 PSI. Turn the geo-system off, and the gauge would pump back up to 50PSI, and shut off. Great pressure at any faucett, or the shower, with the geo off. Water heater will produce great pressure with hot water. Turn geo back on, and not enough water thru water heater again, and trickle at all faucetts again. When the geo was installed, they said that the well produced enough flow to handle flow for geo, shower, and faucetts, and it did. This winter I did notice that the geo didn't seem to be producing the amount of heat that it did when first installed, and my aux heat was coming on frequently. Now my questions:
Since my well pump is about 22 years old, and only 1/2 hp, is the pump going bad, or is there something else I should be checking first??
Should I upgrade to a 1 hp pump??
I am a do-it-myselfer, so is this a job that I can do with the help of my neighbor??
If I can do this myself, is there anything else that I should do besides pull it up, screw on a new pump, and stick it back in the casing??
I really would love to have it done, but with the money and job situation the way it is, if I can at all possibly do it, I have to try.
Oh yea, the house has all coper and PVC plumbing, no galvanized. With geo off, all faucetts work great, pump cycles normally. With geo running, pump never shuts off, but it did when first installed. Any help would be greatly appreciated. I don't know much about wells, but I think that I will be in for a crash course on well repair. Also, is there any pump brand that I should be considering for my situation, and any thing that I need to be looking for in the pump that I do buy. I really don't want to have to do this again. Thanks again for any help. ( my wife thinks that I can't do this,, damn, I have to prove her wrong)
Dave

It sounds as if too much water is going through the geo, as if a flow control is not working. And your pump is doing fine to keep up without the geo, so there's nothing wrong with the pump or the plumbing it is on in the well or underground to the pressure tank.

So check the geo for a busted line, bad flow controller of some kind etc..

BTW, when it comes to pumps, bigger isn't always better and you don't size a pump by hp alone. You first establish how many gpm at whatever pressure you need and then find the horses to do the job of delivering them.

Increasing horse power will only increase your electric bill. Where does the geo system dump? May need to do a pump test.

The geo dumps into a open loop line that is about 4 ft under ground. the line is about 120 ft long. The ground is mostly sand. How can I check the amount of flow going thru the geo?? It does have two ball valves in the water line, one before it goes into the geo, and one where it comes out of the geo. Those seem to be open about 45 degrees.I didn't see any flow limiter or flow control in the line.
Dave

Then maybe a call to the geo guys sounds appropriate.

Or, if you could use a possible adventure, index the outlet valve and close it some and see what happens to water flow in the house with the geo running. There could be something wrong with the valve, like a worn ball. I'd call the geo guys first though and ask them where the flow control is and what happens when you close that valve some.