Heater problem

[riander4]

Ok so here is the deal.
91 240sx sr20 swapped, but having a heating issue so i believe its nothing to do with the JDM engine

When im driving car usually heats up fine (engine water temp gauge sits where its supposed to). Heater blower working, but no heat since it got like -30 here in saskatoon. Took it to the mechanic... he said he got heat going, after it was sitting in the shop for a bit. I washed it today, and it had heat after i washed it and it got to warm up for like 10 minutes. 5 hours later i drive it again and no heat at all.
Also.. If i am driving and have my heat on full and blowing at the max (but no heat coming out only cold air) sometimes the car will overheat a bit, by about a bar or so.

So here is what i think is happening... maybe coolant i put in wasnt rated for this cold, so its freezing (semi freezing) somewhere between the engine and my heater core, so im getting no hot air, and is occasionally plugging the whole system up so for a period the engine is getting no new coolant, therefore the overheating problem.

and car has a new water pump as of the summer, and dual electric fans going all the time.... so theres no way that not cooling / water pump issue

thanks in advance