My car goes into the low gear and reverse

Ok my car only goes into reverse and low the L on the shifter and was going into high Rpms to shift when I first bought it. After a few days of driving it the reverse and neutral went away and it was like drive in everything but park and drove good just didn't have reverse or neutral like I said it was like drive but the Rpms went back to normal when it shifted. I changed the fluid and filter already before this happened. And I work at the bowling ally as a transmission maintenance man for bowling machines. So my boss who is the transmission machanic their told me it's probably the transmission modulator going bad. Well today it was like I ran over a bump in the road or something then it went back to having reverse and neutral and L low gears again just like the first day I have bought it. Do you think it is the modulator or shift silinoid causing this problem.