Nissan Murano 2003 overflowing coolant reservoir. help us with this mystery, please!?

Hello. I wrote in yesterday, so i might have a repeat here, but it is not appearing yet.
My Nissan Murano, 2003 is acting up after a ton of diagnostics and repair . We replaced thermostat, flushed heat core, replaced coolant pipe which had rusted and leaked out fluid, and ran air bubbles out forever, it seemed. The heater does not work now on lower rpm’s and the car was gurgling the other day when i stopped at mailbox. I have been driving it for a few weeks since we had it repaired.
When i pulled in into garage, it was overflowing at reservoir for coolant. We have not driven it and our mechanic is perplexed. He ruled out head gasket problems with chemical test and another diagnostic test. . . .he is perplexed. He added coolant and what else. . … .that is all i can think of . should we get it towed to dealer which is 100 miles away? we’ve already spent 1500 or so with this and it seems we could be back at square one. Our mechanic is very honest, and he does think it could be in the engine, like a warped plastic piece. . . i don’t know what he said exactly, but he’s just guessing.
We have 128k on this engine. Thanks a lot - See more at: http://community.cartalk.com/discussion/2290902/nissan-murano-2003-leaked-coolant-rusty-pipe-stalled-fixed-pipe-now-what-please-help#latest

Has the radiator cap been replaced?
It would cause many of your symptoms if it’s not holding pressure.

What was problem #1? Low coolant due to rusted coolant pipe? And ur at $1500 in bills to fix that? Low coolant will cause engine damage. See all the posts from Subaru owners here? So motor is still overheating now. Cooling fans are running? Hmm, coolant level is fine, heater should be blasting hot air? Yes?

thanks, yes, circuitsmith, it was replaced with new one.
Stoveguy, rusted coolant pipe burst and left coolant on ground, apparently. I drove Murano another mile, when it stalled again and i had it towed to one facility that flushed heater core, tried to get bubbles out, and replaced the burst pipe. they then thought it was needing head gasket repair, and do not do that work, so our mechanic friend took it to his shop and replaced pipe(again), and flushed heater core, replaced water pump(which was apparently fine, he said later)… .and replaced radiator cap,. . . .added much coolant, thermostat, and ran chemical test for head gasket but found it to show that it was fine. Hmmmmm. . …thanks

He added a 180 thermostat since he couldn’t generate enough heat. . .he says