Our la cimbali m29c12-mf heats up for a few minutes then cools down then heats up again. What is the problem?

Low pressure or luke warm tank water is a problem with the pressure switch (Pressure-stat)
You should contact your vendor for service. Also contact your coffee vendor for advice.
*** The following is for information only. Repairs should be done by a qualified tech.***
* Pre-mature heat cut off may be a faulty pressure-switch. Check for broken spring, clogged (lime) pipe, broken micro-switch)
* Problem with the heating system. Check power relay (contactor), Solid state switch. Replace.
* Loss of phase (3 phase power) or loss of power to one or more elements. Check electrical load on each element with amprobe meter.
* Loss of power on outlet/plug. Check outlet and plug for burnt or loose wires. Check wires and re-snug down connections.
* Over-filled boiler tank. Check water level probe or water level float. Low or slow heating due to water expansion when water is warmed but not enough to boil. Drain some of the boiler water and try again.
* Wrong voltage on power line. Do you have Brown outs? Black outs? on city power. Plug into correct outlet. Don't use extention cord.
* Tank is old from long use (+3 years). Boiler is full of lime crystals, sand, debris. Needs tank overhaul or replacement.
* Final, If you have more than 1 of these problems, then you should have your machine checked. It probably needs servicing.
Aloha, ukeboy57

1Check the inlet cable of electricity 2 Check the heating element and heating element controllers (thermostats).