Lightning Riders

I am the proud owner of a new Phanton. At my ancient age I go up hills slowly. I am especially impressed about how one can power the Phanton up hills at speeds low as 3.5 mph and track a straight line. This characteristic is in contrast to my former LWB's (EZ-1, Limbo, Canto) which wandered side-to-side at low speeds.

But ... my wife and I will be going back this summer as usual for 6 weeks at our place in the Adirondacks where the riding is on dirt roads only. I tried a Big Apple 20 x 2.0 (60 psi) tire on the front wheel (without changing the rear tire) and the bike then wandered at low speeds. What's going on? Any tire recommmendations?