Heil 7000 furnace, blower stays on and burners don't ignite

Furnace Info:
-Upflow Condensing Gas Furnace is a Heil 7000 (model# NUGK075DG09)
-Electric spark or host surface ignitor ( I think it's spark)
-Installed Oct. '96 (when we built the house)
-90% efficiency
-wall thermostat is honeywell programmable. Don't know model but was also purchased in fall of '96


Problem:
I noticed the blower on the furnace hadn't shut off and was still blowing air when it should be off (thermostat fan setting was "auto").
Later, when heat should have been on, I felt cool air blowing out of vents, so then realized it was not blowing heat.
Checked out furnace and burners were not lit.
Then shut off power to furnace so blower would stop.

Called local service. According to my wife who was home, service tech stated he couldn't find the problem and that the furnace started for him.
He then showed my wife a limit switch that could be used to ignite the burners by touching what looks like a small post on the switch. My wife thought he used a screwdirver to touch a post on the switch. I'm not exactly sure which limit switch it is. It is above the burners. I have the manual, but it isn't real clear to me which switch it is (there are 3).
I attached a couple of images with the limit switch circled in red.

By the next morning, furnace wasn't working again, blower was constantly on and no heat.
I used a screwdriver as indicated by the technician on the limit switch to see if the burners would ignite. When touching the switch, it sounded like the exhaust fan started, and a few seconds after that, I could see the spark ignite the burners. But, the burners don't stay lit. As soon as I take the screwdriver away from switch, they turn off. THis was done when the thermostat temp was set high enough that the furnace should be operating.

The description on the invoice left just stated "fix limit on furnace & check operation".
Tech had noted if you had to start the burners more than a couple times by touching the limit switch, we probably need to get a new furnace. That seems a bit extreme and wonder if some component needs to be replaced.

Any suggestions? It's starting to get cool here in and, so need to figure out soon if I should make another service call to see if it can be repaired or start shopping for a new furnace.

Thanks in advance!