4 new LED spots, 3.2 watts each (sealed: cannot replace bulbs). Lites and transformer labeled for 12VAC. On a GFCI protected, photocell- and timer-controlled circuit (photocell turns lites on, timer turns them off after 6 hr). Usually 3 lites (landscape), and sometimes add 4th lite for flag. When 4 lites, timer is bypassed so lites stay on all night. All lites are 4 to 25 feet from transformer and on 12 gauge wire.
Setup works fine with older, large transformer designed to handle much larger, halogen load. For esthetics, I wanted to replace with small electronic transformer (12VAC, 10 watt min, 60 watt max) that would fit in standard electrical box on same circuit. New setup works fine with 3 lites (even though 9.6 watts is slightly below transformer minimum), but with 4 lites (12.8 watts) starts flashing after about 15 seconds then goes out after about 30 seconds. Switching back to 3 lites and it works fine again. I did switch out the 4th lite for a 10 watt halogen. Appeared to work fine initially, but later on in evening all lites were much dimmer. Transformer supplier says it has been tested with their lites (including LED) with no problem, but had no answer for me. I am not using their lites. Any ideas?
Tyson asked about the photocell - it is a three wire photocell. And a correction - the low voltage cable being used is apparently 18 gauge, not 12 gauge.
In answer to Harper, the 4 lites total only 12.8 watts and the transformer is rated for 10W minimum and 60W max , so I don't know how it could be overloaded.
As described previously, the system worked fine with 3 lites, closest of which was 12 ft away and two more were another 11 ft away (Y'd from the closest one).
I have done some more experimentation > The lite which (when switched on) caused the problem was only 4 ft from the transformer and switch. Too close? So I connected that particular lite instead to an end that was 23 ft away (as described in the previous paragraph), with a cable that was about 32 ft long, so total of 55 ft of cable with two lites between the transformer and the problem lite. Then it worked, without flashing. Makes me think that being too close to the transformer caused the problem, but that's where that lite is, attached to the same wall as the electrical box that holds the transformer - not out in the yard like the other 3 lites. Then I connected the 32 ft cable to the transformer leads (along with cable to the yard lites that were already connected). All 4 lites work without starting to flash. So the transformer has a nominal 12.8 watts of load, but the lite that is 4 ft from the transformer is now separated from it by 36 ft of cable. I don't know if the length of cable increases the resistance or helps smooth out the transformer's "square" sine wave (apparently an issue with LEDs and electronic transformers). So instead of the extra 32 ft of cable (which I don't want on the front patio), is there something I can use to equivalently electrically isolate the offending lite from transformer?