Panasonics are indeed notorious for WIFI problems.
Their most advanced 2.4 ghz line starting with KX-TGA270s have no problems with wirelessm, but the HUGE caveat is that they use Nicad batteries!!:Q :Q :Q As expected, users have TROUNCED this Panasonic line for really, really crappy battery life. See amazon.com reviews on this issue.
The Vtech 5.8 ghz high end stuff is acclaimed for NOT messing up the wireless. An all out great product. I was biased against Vtech, but seems they really nailed it here with this product. I really like the speaker phone feature on the handset. Can be purchased for $139 @ Amazon.
The Uniden 5.8 ghz high end stuff also messes up the wireless. I was really upset at Uniden. They generally have great phones at sensible prices, yet alas, the 5.8 ghz model knocked my wireless off kilter. It is the only 5.8 ghz phone that transmits both ways at 5.8 ghz. The other competitors transmit 5.8 ghz one way and 2.4 ghz the other way.
Alas, for wireless, go with a Uniden digital spread spectrum 900 mhz, Vtechs 5.8 ghz, or hard-line :Q .