83 300D Glow Plug Question

After a cold start, it runs rough for 1/2 minute before smooting out. Also, lately the dash glow light has not been working - I thought the bulb was out. I have the pen type glow plugs and tested their resistance at the glow plug connector (at the timer relay). All had similar resistance except one which was much higher than the rest. Interestingly, while the glow plug connector was disconnected from the timer relay, I turned on the key - and the dash glow light worked. I re-connected the glow plug connector, and the dash glow light stopped working again. Weird. Ya'll please help. Thanks.