same trouble with my '05 V6 4.0L SOHC
I am having the same problem as you on my '05 V6. I've replaced the cap and the thermostat myself and still not fixed. I am about to replace the ECT sensor, Tstat housing next.
I tried the block tester myself and it came up negative for exhaust gas but I am not so confident in the test. You can borrow it from autozone for free.
let me know how you make out. I'm concerned of a HG crack on mine and am not looking forward to the $$$$ it will cost to fix.
BTW I ride at sea level here in NJ so I don't think your altitude is an issue.