2003 Suburban Z71 suddenly blasts hot air upon acceleration

Within the past two weeks my 2003 Suburban will suddenly blast hot air when I accelerate. Usually happens when the a/c is on and I am merging onto a highway. The 'burb has nearly 147,000 miles on it and has been a real work-horse for our family. The “low coolant” message comes on randomly. Had it checked by the dealer and another mechanic who agreed it is a faulty sensor and that the cooling system is fine.

My husband insists it is just the a/c but I remember my grandfather had the same issue last summer and it was something big.

I drive this for my business and am often on the road at night alone and really do not want to break down in the middle of nowhere!

Thanks for any and all suggestions!

Good vibrations,
Cat