My air conditioning nightmare.2000 Grand caravan

Yaaa,the ac is working,sort of...I ordered the new line some time ago.the wrong one came in so it got reordered.It finally cam in on tuesday,got installed today. The ac blows very cold,but only at highway speeds. I'll assume this is due to pressure bilding up in the system and a lerger heat transfer at the condensor. The ac works at at idle in the parking lto but not as well. There is a hissing noise coming from both evaps sometimes there and sometimes not. at times the ac works not at all.I believe the h valve needs to be replaced.They are cheap so why not...