Have a wiggle of the headlight connectors, these play up big time, especially the earth connector in the plug, mk1 sierra's seem very bad for this.
On my dads xr4i I relayed the headlights, made a monster difference all on it's own, then add Philips Extreme H4 bulbs brings it right into modern day performance, wouldn't bother with HID, just taking it off a car I put it on as it was better on Extreme, by a long way.
tabetha