Hi,
Have you got back EMF diodes across the coil terminals of your solenoids?
What MOSFET modules are you using.
PLEASE get out pen(cil) and paper and draw a schematic of your complete circuit.
Do you have Mega gnd connected to gnd on the MOSFET modules?
Do you have a fuse in the postive line of the battery to stop the smoke coming out when you have an overload?
Tom... 
