Cannot get Desktop on Wireless!!!

Hi. Hope everyone is well!

* I have a rather infuriating problem with my desktop computer.
* Ever since we changed ISP's it has had terrible performance on the wireless.
* I am connecting with a usb wireless receiver (dongle), to a WPA encrypted home wireless router.
* I have checked all the settings I can think of in the computer and it all looks fine.
* I changed the wireless usb dongle to a new one bought off ebay which improved things but still very slow and lots of disconnections.

Please can an expert give me some idea's of data to check to perhaps get a better idea of what is going wrong, then when I call Tesco broadband I can hopefully speak to 2L engineers and explain exactly what is happening.

Its strange because sometimes it works alright, driver re-installs can make a difference but still inconsistent.

Any help would be much appreciated!

Dom..