Re: Engine stumbling when blower motor turned on
Sounds like your alternator is charging and there doesn't seem to be a draw over time. If it's sitting over time and starts fine, it has plenty of juice to run the ignition.
Check the connections and what type of voltage your using to power your ignition.
If you have a factory (or close to it) alternator, it could be that your powering your ignition system with a circuit that was meant not to be shared. Or you've tapped into the ignition circuitry to run the accessories.
Either way, it sounds like your using up the voltage in your ignition circuit, and it's not able to access battery voltage to make up for the large draw.
Have you had your alternator checked yet? Have you been able to or had someone else watch the headlights as the engine dies? Do they dim or go out? I have heard of alternators that charge fine under light loads, but give up under heavy use, but even if this was the case, your battery should be able to run things. Keep at it.