Boosting 18 V drill battery to 60 V to power LED lights, problems with voltage stability
I believe that your batteries may be too small.

I use modules that look very much like (probably identical) for my LED yard lights.  However, my lights must be smaller than yours - mine are 38V @ 1.7 Amps.  I removed the original power supply and use these modules instead.

The smallest battery I use is Dewalt Flex-Volt batteries - I have both 6Ah &amp; 9Ah at 20 Volts.  Do note that Dewalt plays a little loose with their battery voltage ratings - most people would call these 18V packs rather than the 20V rating that Dewalt uses.  

My battery packs are 3P5S with either 2AH cells (6Ah) or 3Ah cells (9Ah).    These packs can be switched into 60V mode by means of a mechanical switch contained in the battery pack but I'm using them in 20V mode.

My portable lights work extremely well with those power supplies.  Do note that I am using the power supplies in Constant Current mode.  The open-circuit voltage is set to be a couple of volts higher than the maximum voltage that the LED lamps will ever require and the current is set to the proper value.

I've also used those modules to power 4' LED strip lights (fluorescent bulb replacement).  I'm using one module per lamp, directly feeding the LED array inside the tube.  In other words, I'm not using the internal power supply within the tube.  Also great performance using the Dewalt battery packs.