Hey Denny,
Sounds like the freon is getting a little low...When the rpm's go up... the compressor spins enough to make cool air,, at low speeds,, there is not enough freon to keep cooling,,
If you are mechanically inclined, you can use a A/C Pro..

https://youtu.be/ayV1oEo--xA


https://www.target.com/p/ac-pro-refrigerant/-/A-14777981?ref=tgt_adv_XS000000&AFID=google_pla_df&CPNG=PLA_Home+Improvement+Shopping&adgroup=SC_Home+Improvement&LID=700000001170770pgs&network=g&device=c&location=9012720&gclid=CPOw562f4dQCFUM6gQodMKkHRA&gclsrc=aw.ds

https://shop.advanceautoparts.com/brands/ac-pro

Pro Pro 134a pro premium Not for Hybrid vehicles with electrical driven...

http://www.oreillyauto.com/site/c/ACProRefrigerant.oap