What type of riding are you doing?
I have been using a Splice Super for more than a year and its performing really well.
I dont know if its due to low expectation or my lack of experience with higher quality forks (Rock Shock Revelation is the best I have used so far. In the world of high end Foxs and Marzocchis thats not saying much).
I found the Splice Supers to be very responsive for trail riding (no big jumps or drops). they were very plush and I could use almost the entire length of travel. My fork came with travel adjustment, rebound adjustment but no lockout or preload adjustment. The only thing I changed on the fork is, I put firmer springs (the fork was too plush and was sucking up energy on long climbs).
I also found that correct rebound adjustment helps Splice's performance substentailly. So, you may want to check on that. In my opinon, Splice Super is a very nice fork for trail riding, especially if you are talking about performance. If you want to add the bling factor to your bike go with Marzocchi or Fox. They may perform better too but you have to pay lot more money for that.
So, do some adjustment on the fork, talk to a local bike store and possibly Manitou about the issues you are encountering....and there may be some solution to your problems. Marzocchi MX series is pretty nice, but why pay that much money if you have a decent new fork to begin with.
good luck .