Stopping the machine completly stops after each stitch and I have to keep depressing the foot control

Try to solate the problem to machine or foot control by detaching foot control and use stop/start. If stop after a start exhibits same as foot pedal then likely the problem is machine control board else, foot pedal. Could be foot control is sticking and not completely opening or slow to open.

I assume you are using the foot control not the Stop/Start button?

Simple straight stitch or a programmed pattern?

Could be foot pedal issue, so try using Stop/Start and a simple straight stitch. Or could be a programming error or other so I suggest you isolate any foot pedal issue by using Stop/Start and simple straight stitch. Reply here with results.

Might try a different presser foot. Could be the machine is not reading the current presser foot correctly or the stitch selections are not compatible with the current presser foot being used, ie a straight stitch presser foot while the stitch selection is zig-zag.

it's very possible that when the needle broke the machine went out of time & would have to be retimed at a sewing machine shop

Go to singerco.com and under customer support, look at frequently asked questions, they have alot info there. If, that doesn't work for you, call customer service at 1-800-4singer and they can advise you

Call Brother customer service at 1-877-276-8437 and threy will help you

Double check the instructions and if there is still an issue, call the Bernina dealer and tell them what is wrong.

Ask if someone will be available to look at the machine when you come in .... you need one-on-one assistance.

You could have an electronic problem on the main printed circuit board or in the feed length stepper motor. You may also just have a bound section in the feed mechanism. I don't think there is a cure all answer for this problem.
A 1/2 second hesitation between depressing the foot control and motor response is very common in these machines
If your husband lubricated the bushings on the upper and lower shafts of your 990 I recommend that you have him remove all of the lubricant possible and keep your fingers crossed. The 990 is a self-lubricating machine and applying machine oil to these bushing will probably cause them to seize. The 990 should never be lubricated in many of the places you would lubricate many machines. Hopefully, you haven't done irreparable damage.

Greetings, I experienced the same problem. Mine only ran a few stitches. Foot pressure on the foot feed was always "tension" problem in my foot.

I turned the foot feed around and upside down.
I press on the foot feed at the open end not the pivot (cord) end. This has given me much better control. Janome had no options for foot feed adjustment or replacement, but this has helped greatly. You may have to practice to get used to it

this sounds like its set for zippers check settings!