Laars LX400 heater - burns erratically and fries fusable link

Hey all,
My albatross of a heater, a Laars/Jandy LX400 low NoX model of a about 5 years is acting strangely again. At the beginning of the season, I had to replace a number of the burners so that it would light and burn consistently without giving me an AGS (Automatic Gas Shutdown) error. Thanks to all who helped from this site! You all pointed me to the burners - I replaced the rusted, damaged ones and voila! It worked great all season until now. I noticed the heater had shut down with the EXT 1 error - which is either the fusible link or the flu temp sensor. I checked and the link was open. I replaced it and watched it try to light and FOOM! It lit with a big boom and a fireball and all the burners were alight with blue flames dancing. It was pretty scary. I shut it off, closed it up and tried again. It lit with a FOOM! and ran for a while until the fusible link opened again.

So - I don't want to blow up my pool house.... any ideas? I don't think the burners are bad again - they are all new. I probably should take them out to be sure. I inspected visually and didn't see any spiders.. ugh!!

I wish there was a pool tech I could call, but alas, no one will make the trip.

I hate to sound so desperate, but HELP!

Thank you, thank you, thank you in advance.
...heather