Halogen Class 2 Power Supply - Transformer

[tafhelp]

I hope someone can lead me in the right direction. Recently a few of my under cabinet halogen lights started going out after about 8 to 10 minutes. I checked the bulbs and they are ok. When the kitchen was re-modeled 7 years ago there were 3 Halogen Class 2 power supply units installed in the basement. Each of them is rated at 120V 50/60Hz input; 12v 65W/max output. I have twelve under counter lights each with 20w halogen lights. There is one area of the cabinet lighting where 4 lights are going out after the 8 to 10 minute period of time. The transformers are warm to the touch.
My questions are -
Do transformers have a lifespan?
Is it possible that the transformers were undersized to power 4x 20w lights?
If the units are undersized, is it ok to simply replace the transformers with higher output transformers?
Thanks for everyones help.

 

[tafhelp]

closing update

Just wanted to close this off...
I have replaced a fourth transformer I found in a junction box beside the other transformers. There was a 150w transformer in the junction box that powered the 4 x 20w lights. I replaced the transformer with a 105w transformer. In conversation with electrical people they recommended a 20% overage capacity for the transformer. I was concerned about replacing the 150w with a 105w but at the moment no problems and everything works again. The little women is happy again.