Missing/Dropping X or Y steps randomly

For a while I thought my head must have been crashing, but I caught my printer pausing for 500ms and then continuing as if it had dropped a few steps.. at one point I was looking at pronterface and the line count was going up but my head wasn't moving. It's random when this happens short or long prints. Where do I start with this... get a sd card to and print off that for a while to confirm fault?

Thanks.

I haven't had that but I do get random disconnects - it seems that the comms is somewhat flaky which is rather poor in this day and age.

Printing from SD should show whether it's a comms problem or not and it's easy enough to try so that'd be my first step.

Is adding sdcard support as easy as ordering one from reprapworld, soldering it on my ramps 1.4 board and changing a line in my sprinter firmware? I was so close to ordering sd last week but I couldn't think why I would need it. I didn't have this problem then.

Thanks.

If the layers move, you're missing steps. If it's just pausing (this is called 'segment pausing'), and you get a blob, it's probably that either:
a) The gcode isn't being sent fast enough. Check if there are other USB devices that are interrupting sending to the Arduino, try using a faster baud rate to communicate (if you're on Ubuntu, you'll be limited to 115200), make sure other processes aren't interrupting communication, make sure electrical interference, ie from motors, isn't interrupting the USB (increases error rate - get a shielded cable). Unfortunately, the memory buffer in most firmwares is set quite small, usually only about 8-10 commands are buffered on the Arduino, so if there's a slowdown or interruption in the gcode sending, the buffer empties and it has to wait for more to be sent. I have found Ubuntu the worst for this; Mac OS is better, and I need to try Windows again to see if it copes; of course, it's helpful that these two can communicate at 250000 baud.
b) The gcode isn't being interpreted fast enough on the Arduino, probably due to lots of small, fast moves, as the acceleration has to be computed for each move. Make your model simpler (less facets), and/or reduce speed. This will also help the first point.
Otherwise, yes, try printing from an SD card. This will highlight if it's problem a or b!