My heat normally only works on the highest settings, but when i'm driving over 45 for longer than 5 minutes, it works on every setting. Once i stop it goes back to only on thr high setings, and if i put it on anything other then floor it cUses my rpms to surge. Ive bled the coolant system hoping for a fix, but nothing, i was wondering if maybe the water pump could have something.to do with this, or could it be something else? Any help would be greatlh apprrciated