Frigidaire Gas Oven FGFL88ACA Heating Problems

This Frigidaire Gas Oven FGFL88ACA was installed at our home when we purchased it. It has worked fine for the past few years, but now when you set a temp it may work fine, or it may never reach the set temperature.

When you turn it on there is a time lapse of between 60 and 100 seconds before the ignitor fires (that may be normal for all I know). The intermittant nature of the behavior is making this difficult to diagnose. Any ideas?