I had the same type of problems with a 18F2550. Sometimes it functioned, sometimes it didn't!
After searching and reading the 18F2550 manual i found the chapter
"17.2.2.4 External Pull-up Resistors
External pull-up may also be used. The VUSB pin may be
used to pull up D+ or D-. The pull-up resistor must be
1.5 kΩ (±5%) as required by the USB specifications.
Figure 17-3 shows an example." and soldered both the resistor and C and happiness it functions. The pullup resistor for D+ to Vusb (1.5K) did the job for me.

Best regards,
Onno Tromop