Potential issue with thermostat wire length

Hi, I recently had an issue with my Goodman brand furnace. The direct ignition pilot would not light (or lit for a second and then went out, lights for a shorter period than if thermocouple is disconnected or not working). We had a service tech come out and he said there was an issue with the control board and replaced it with an ICM Controls 286 board. Everything worked fine for a few weeks and then we had the same issue. After some of my own troubleshooting, I confirmed the furnace would always heat if i removed the thermostat and wiring and jumped R to W. If I brought the thermostat downstairs and connected to a 10' section of wire, it would work fine. I pulled a new cable upstairs, 18/5, and reconnected upstairs. Everything worked again. For a week! Last night, same thing, heat would not work again as pilot would not light. I removed the thermostat upstairs and wire-nutted the R to the W and the pilot still would not light! Brought thermostat downstairs, put on my 10' section and it works fine! I tried a programmable Honeywell thermostat this afternoon, battery powered upstairs and that wouldn't work either. I pulled the my upstairs wire run downstairs in case of interference in the wall and the thermostat still wouldn't work on the longer wire run. In all cases, when this happens I get 8 blinks on the circuit board. I've connected the G terminal to a good unpainted spot on the furnace chassis. Any ideas? I'm stumped. Thanks in advance.

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