Need help steering box? Car out of commission

I have a 74 Dart Custom that is having steering issues. All ball joints tie rods etc. have been checked by myself and 2 mechanics and all are good there is only slight play in one upper control arm bushing. I have replaced the wheel bearings but one may be a little tight. When driving the car the steering wheel tends to be at 45 degree angle but while driving down the highway at times typically when hitting a bump the steering will jar hard to the one side after pulling it back (it has jarred more then once in a 10 second period at times) the steering wheel tends to straighten out and drive perfectly straight down the road in a straight position (slight pull to the left but very slight) but then after 20+ second the wheel will slowly need to be put back at the 45 to keep straight but it is gradual not sudden as when the car jerks to the side. I have had 2 front end allignments done recently the second one because a 92 stealth hit my front left tire. But the car was doing this before just not nearly as bad. The only thing I can think of is rebuilding the steering box but its 90$ for parts or 290$ for a rebuilt box and it looks like a pain to rebuild or change (have pulled a few to sell just never installed one) I have 2 of these cars both the steering wheels are on 45's I am worried if this has happened to one the other is not far behind. Both cars are daily drivers and drive 140km's a day to work and back and my wife and kids ride in them since they are my only cars at the time so I need it fixed properly and hopefully not to expensively.