with the memclock dropping to 150 instead of whatever you have set it at, every time you attempt to mine a coin, it spikes up, causing it to run at the speed you set for that duration. problem with this is that it creates a delay and ultimately slows down your hashrate from what it could potentially be. proof is in the pudding to be honest. i tuned my 290x up to 1200/1500 for a few mins to see the hashrate (don't expect it to be stable because i haven't modded voltages) and it hit 3.8MH on x13 and stayed there until my system froze. it just helps.
(anything i say is just what i see happening from my POV, if it is inaccurate then apologies. same result in the end though!)