HVAC blows hot/cool, then lukewarm for hours

I have a 6 year old Goodman 3 1/2 ton "split system" heat pump; the compressor is about 40 feet from the air handler. The heat comes on, blows hot (108 degree) air off and on for a few minutes, then blows lukewarm (78 degree) air for hours, seldom reaching the thermostat setting.
I've got a graph that shows the pattern over a couple of hours:
http://img138.imageshack.us/img138/2423/hvac2ac5.jpg
So, with the thermostat at 70, the air in the vent starts at 64. Within 10 minutes, its over 106. Then at 25 minutes, it drops precariously to 76, then immediately returns to 106/108. It stays at 106/109 for another 25 minutes, then drops again to 74. It spikes a time or two, then settles in at about 78 for hours, with occasional spikes down to 74 and up to 88. The temperature in the house never reaches the target of 70 degrees, hovering around 67/68.
My repair guy (who first sold it to me) seems unable to remedy the problem. About yearly, he checks/replaces heat strips, rewires and replaces the thermostat, adjusts some differential, and declares it working. But any meager improvement seldom lasts very long. He says the drops and spikes are normal.
What's your take on it?
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Do you have a digital thermostat or mercury bulb thermostat?

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wrote:

Replace the stat. You've got a bad one. "Its almost always the thermostat" Bubba
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If you could graph the refrigerant pressures and compressor amperage draw it may indicate a refrigerant side problem such as leaking valves or overload tripping intermittently.Even the temperatures in and out of the outdoor unit, over time, may be helpful. Work with your " repair guy" only if he is competent, qualified, certified, licensed, insured and all those other things, if not, find one who is!
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I have no idea how I'd measure these. Can the repairman?
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DrYattz wrote:

Dr Yattz;
Sounds like the Electric Strip heaters are cycling [either because of defrost or 2nd stage on the thermostat.] I'm guessing, but I'd say the refrigerant charge may need to be checked. Your symtoms seems like the 2nd stage on the thermostat is cycling the strip heaters. [Runs up the electric bill considerably.] If the outdoor unit is not performing, it may be either colder than hell out, or you're low on refrigerant. Maybe a call to another HVAC repair company might be in order. I'd check with your family, friends, neighbors about who they've called in the past and had good results with.
--
Zyp



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It's lasted 6 years????!!!!
; the

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