I had the old navman 3420 sleeve for my ipaq 3950 and hated it. It took 20 minutes(if i was lucky) to acquire a crappy signal on a clear day. I tried several fixes for this (first putting aluminum foil between the ipaq and the sleeve which didn't really help). I also looked for a mod to install an external antenna but people reccomended using a re-radiating antenna (which is an external antenna attatched to a transmitter which reradiates the GPS signal through a wire which you loop around the acual GPS receiver you have (the navman) This worked great but came with too many wires and eventually broke. I believe the navman 3450 sleeve has an external antenna but people said it didn't acually improve the signal all that much.
After giving up for two years I bought a Magellen Meridian portable gps for other reasons which has a serial port connection. This past summer I bought a serial/car charger adapter for my ipaq and successfully used my portable gps with my Ipaq using TomTom5. This was a godsend. Since I was using an independent GPS i never had a problem acquiring a GPS signal and the TomTom software is just phenomenal.
Oh and I also got a suction mount for both the GPS unit and the Ipaq although the mount for the ipaq is acually an extended arm with a sleeve that fits over the navman sleeve(i still use the navman to put maps on its CF card). Once mounted and plugged in this setup worked like a charm.
I'd say ditch the navman and go with another solution. You could get the serial adapter, TomTom software, two mounts, a GPS unit and the necessary cables. However, for the price of all this I'd reccomend getting a TomTom standalone unit. But if you already have half the stuff lying around like I did it's a worthy option if you don't mind setting it all up. Cheers.