Sounds like arc blow to me. It's not uncommon in tight areas where the magnetic forces are concentrated. dbigkahunna, aevald, or some of these other Doctors of Weldology can probably tell you exactly how to minimize or eliminate it all together.

Your heat is about right, but try using a forward angle with your rod, (dragging). If you keep your rod tilted forward and your heat is right, you should be able to keep your puddle just right, instead of overtaking you.

Also, if your coupon is less than flat, you could be slightly down hill, which is usually going to create the problem you're having. 7018 is actually a multi position rod, but unless your heat is crankin and you're moving out, it's not a good scenario. Try keeping your work just slightly uphill, even if it means starting at the other end of your coupon.

One more thing: pay more attention to what your arc is doing, because you could be long-arcing and allowing the puddle to scoot right under your rod. The closer you are with your arc, the less that puddle will want to move under your arc. Simple physics.....just quit worrying so much about sticking the rod, and use a little more heat if you have to.

I'm on a lot of jobs where I'm burning boxes and boxes of LH rod, especially horizontally, (mud tank builds, etc,). Low hydrogen rod will almost run itself at a forward angle, if your heat is right. Before you know it, you're out of rod and the sun's going down.....

If it is infact arc blow.  Try starting up at the end of the plate and terminate in the middle and then strike up at the other end and work your way to the middle tying back into your previously deposited weld.   You can also try wraping your leads around your work piece four or five times in one direction or the other (depends wether it positively magnitized or negitively magnitized) Atleast thats how we cancel it out on pipe.

Okay, Interrupting my vacation to throw in my own two tin pennies worth here...

If you are using 1/8" 7018 electrodes to develop a 3/8" Fillet weld in a Tee or Corner weld while practicing "weave" techniques then that in itself should explain the problem.  The slag is coming around your arc because you are really pushing the limits in every way.

1) Use 5/32" electrodes instead and run at about 150 amps. 
2) The electrode should  be angled slightly behind (dragging the weld pool) from the electrode holder and your direction of travel.
3) Use a very slight weave not a lot of movement side to side (many people think of 'weaving' as being quite a bit of side to side movement that actually allows the slag to come around your arc and then cool too much before you get back to the other side) thus keeping the weld pool and slag very molten as you 'weave'.  The arc force will usually push the molten slag out of the weld root when it is hot enough. 
4) When welding tee and corner welds you may even want to be pointed straight into the joint instead of the slight drag and as you weave use a 'SLIGHT' triangular motion that burns into the root good, similar to some of the vertical up movements with a hesitation on each side and up into the root.  (I did say 'similar' to, it won't be exactly the same).
5) Keep the arc short and tight, I can generally FEEL the flux dragging on the steel as it is burning off with the electrode. 

Without seeing and/or testing your pieces it is hard to know if you are indeed trapping slag under the weld in the root of the joint.  But I would suggest these changes to reduce the possibility of that happening.

Have a Great Day,  Brent

LHouage,
you are carring to much material. A 3/8" fillet, flat with 1/8" E7018 is to much to try to do in one pass in the flat position. Try multiple passes. Typically a fillet weld that size would be at least 3 passes. My opinion of coarse.

Jim