If a different guage shows the same,make sure you dont have any restrictions in the fuel lines. Old rubber lines tend to colapse, check for crimps in the metal lines. I had a fluctuating fuel pressure guage and turned out that the sock in the tank had become 90% blocked. One it was cleaned out, the fuel pressure went steady. Slim chance that is your problem because you would probably experience starvation with that carb, especially with the secondaries. If it keeps evading you, go ahead and check.