The following link explains how to replace a glowbar and how to measure them:
http://www.fixya.com/support/r401858-replacing_oven_igniter_gas_range
The first thing you need to do is determine which type of glowbar igniter you have. The current tolerances are different on each one. If you take a current reading and it is not within the prescribed range, replace the igniter (even if it is glowing). The easiest and preferred method measurement is to use an amprobe type multimeter and clip it to a single wire from the igniter.
Most commonly, the igniter, rather than a gas valve, goes bad after periods of use. In particular, the bake igniter because it gets used more often. In addition, your range should have a separate igniter for the broil manifold as well. Since most home owner's do not own an amprobe meter, you can swap broil and bake igniters to see if the symptoms change. If the symptom moves to the broil manifold, then you know you have a bad igniter. If the symptoms stay the same and then you need to consider the option that you may have a gas safety valve issue. Let me know what you find. I hope you find this information helpful.
PS I'm only assuming this is a bake manifold problem, as you did not say which one you were having problems with. This is the more common failed component, however, because it gets used more frequently.