Yep.. let me get this straight,, does not heat up after heads are done, then heats up a little bit so you take it to the shop, and then it heats up a lot.
I wonder if you have air in the system again. .... follow my logic,
Does not heat up, for 3 months, then starts to go up, the shop says you have a leak on the cross pipe, they fix.. and then major problems again. if the leak caused a little air in the system this would cause your temp to starts to spike. Then when the crossover pipe was changed you got air in the system again.
Try and burp your cooling system, and see if that fixes your problem.