I've been battling a problem with my car ever since I put in my GROM BT

2005 G35x w/ 110k miles
I drive about 50 miles a day.

1st incident
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I was driving to the airport on the freeway and noticed my car went into limp mode. I pulled off at the next exit and limped into a parking lot (less than a block from the exit). I lift the hood and notice the coolant reservoir bubbling over. I tow it to AAMCO Transmissions who works on it for a day and can't find anything wrong with it other than the engine codes. They find codes u1000 and u1001 but don't have the tech to diagnose which wires are faulty so I tow it to Boulder Nissan. They reset the computer, drain and refill the coolant, put 40 miles on the car and she drives fine.

2nd incident (about 5 days later)
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I was driving home (about 25 miles) and as I'm getting off the freeway I notice that my AT check and engine check lights are on but the temp is fine (at about 40%). I limp home (about 2 blocks). The next morning, I turn on the car and the check engine light is on but the AT light is off and LIMP mode is off. I drive it out of the garage and onto the tow truck and bring it to Boulder Nissan again. They find "slime" in the radiator and on the cap but are 100% convinced it is not oil induced (aka not an oil leak). They can't figure out what the slime is but they pressure flush the coolant and get all the slime out. They refill the radiator with coolant, tighten up the grounds in my electrical system and are done.

In between the 2nd and 3rd incidents I went on a long drive to test my car out. I drove 40 miles north on a 75mph freeway, taking her over 100 for a very short section, then took the back roads through a few cities on my way home. No problems.

3rd incident (about 10 days later)
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I'm driving up towards the mountains and my car goes into limp mode. Temperature reads fine. I get off the freeway and limp into a parking lot (a block away). The car's thermometer rises to about 50% as I'm going from a stop to 20 mph but as soon as I step off the gas, it goes back to 40% (normal level). I leave the car overnight. The next day, I come back to my car and drive it 40 miles north to Boulder Nissan. It does not go into limp mode at any point during this drive.

Other thoughts
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I notice that one of the fans in the engine bay is REALLY loud. I've had the car looked at by Boulder Nissan, Boulder Import Motors, and AAMCO. All parties are convinced there are no oil leaks from the engine, no problems with the head gasket, no leaking into the cooling system, no leaking out of the cooling system. Basically everyone says the mechanical is sound. I have noticed that if I'm going about 40mph and I floor it, it downshifts from 4th to 3rd as it accelerates. When I ease up on the gas, however, it doesn't reshift up to 4th. It "sticks" in place for a few seconds before it shifts up to a coasting gear. I've driven 7 cars in my lifetime but this is the first car that will stick after I floor it. I can even lift my foot off the gas completely then press it again and it will keep in a downshifted acceleration gear. I keep getting check engine lights. The car keeps going into limp mode even when the thermometer reads 40%. It's at the shop again. I had a transmission service done on the car by Boulder Import Motors about 2 weeks after I bought it (no problems before then) and then 6 days later, I had my first incident.

What do you guys think??

Thanks!