vardiff can really mess with your reported hash rate, it's all about accepted shares. I've been on some pools with vardiff that set my difficulty to 12000+ and it freaked out my miners and upped my stale rate because my miners could not keep up with the switching of coins in the multipool. There are many factors that can change your reported hash rate but it's not likely that the pool is changing your hashrate, if the difficulty is set to a specific number like 512 for example then your reported hashrate should level out. It will still go up and down depending on your accepted shares.
Read the above post, I agree that power/heat can have a big effect on miner performance. I have mine connected to a battery backup that reports the wattage, when when my miners are having issues the wattage goes down or fluctuates and I have to restart my rig. You can buy a meter to plug your miners into, that might help determine if there is a power issue and also help calculate your power bill. lol
I got a power meter on my miners since day one, don't want get a surprise when the bill comes in :-)
My 3 S1's + 2 klondikies are doing about 1200-1300 Watt
And you have to subs-tract the power costs from your BTC earning else you would fool yourself big time!
Also you have to keep to calculate the moment the BTC rate drops around the power consumption you have to unplug your miners from the wall, there will be a moment your miner goes down the shredder.
