Rep Power: 282

Hey guys, I posted on this a few months ago. My heat wasn't working properly. It would only work while driving. When I stopped cold air started blowing out. Then 2 weeks later the heat quit while I was driving and the temp gauge went up past the H. So I replaced the thermostat the next day with one that locks open if the car overheats. I replaced the coolant and got all air bubbles out. After that the temp gauge never moved but the heat still wasn't working right. 2 month later I get in a wreck and when I get in my car to leave, I notice the temp gauge is up again but I goes right back down as I drive off. So now the temp gauge randomly moves up and down, but not very often. sometimes just a little, sometimes a lot. I took it to Honda and told them all of this. Two hours later they tell me they let it idle for 30 min and drove it for 11 miles and couldn't get it to reproduce my concerns. that it's holding pressure. They said it was probably the non-honda thermostat I put in (after this problem started ) or that it was an electrical problem because my blinkers were fast (hyperblinkers ) or maybe a bad fin on the water pump.....thanks for the $82 go f*ck yourself. What should I do to pinpoint this problem without spending a bunch of money??? Bad fin? headgasket? something else?

(sorry for the length)