I second the recommendation to lay off the grass; when I'm going to visit the spice, I try to abstain from grass for at least an entire day. The only time I tried them together and it worked was back a while ago when we had some stuff that took like a good 20 minutes to kick in, so we smoked some green, then hit the spice. The normal chattiness I get after a spice trip was magnified quite nicely by the high that was creepin' up on me. Wrote a lot that night. Can't do that anymore, any grass I touch seems to kick in almost immediately, been like that for the last 6 months or so.
My $0.02:
I went through a period where I had some bad trips too, probably blasted off 6 times in a couple months and only 1 of them was enjoyable/positive. Looking back and reflecting, I think it was my Ego trying to reassert itself; like it let me try out some of these new substances, but realized it was slowly dieing and did its best to instill fear into my trip, and it succeeded for quite a while.
Spice can be an alien experience; a solid breakthrough trip is almost completely inconsistent with the established, three-dimensional reality that we have enjoyed thus far in life. I think that's why the "ego death" can be such a terrifying experience. Remember when you were a kid, and you learned that Santa Claus, the Easter Bunny, and the Tooth Fairy were all entertaining lies your parents told you? Many people remember that for their whole lives, or "the day the music died", or where they were and what they were doing when 9/11 happened, or when JFK, Princess Diana, MLK, etc. were taken from this world. Sure, these are tragic events, but, for some people, these events take a portion of reality and completely destroy it. Makes life a whole new experience from that point on, and that can be pretty terrifying for anyone.
So for as wonderfully as the human brain and mind/consciousness function normally together, one has to remember that the brain exists in 3-D space, and the mind spends most of its time there as well. The mind adjusts rather quickly to the spice reality, while it takes the brain a while to figure out just what the fuck is going on. Maybe that's why I, and you (it sounds like), and many others have a series of good trips when we're first introduced to this sacred sacrament, followed by a string of bad ones. I think the spice trip can create a cognitive dissonance so strong that it takes the brain a while to actually register it, and when it does, it throws up all alarms and does the best it can to hold onto everything it has experienced up to the "spice time" as the one and only reality.
Bruce Damer does a good bit on ego death in the first 10 minutes or so of this podcast, definitely worth listening to if any of this stuff about ego rings true with you:
http://www.matrixmasters.net/salon/?p=320
I didn't even really think about what was happening to me with these bad trips right away, it took a lot of time and thought to put something coherent together. Take your time, look where you haven't looked before, and hopefully you'll find an answer!
Peace & Joy