erratic timing

[77f250diesel]

I checked my timing the other day using the pulse adapter and an advance timing lite. While I was hooking everything up I had the engine running abt. 1000 rpm for warmup. When I "shot" the lite at the timing marks the advance was all over the place---brought the engine up to 1400 and the marks where dead on at 4 deg.. I do have some hesitation at full throttle----pump heading south ? Thanks--Dennis