One of the reasons I think many arranger players have rhythm problems is that few of them ever let the bass and drums be loud enough in contrast to their soloing voice.
There is always a tendency to want to listen to what you are playing, rather than concentrating on the rhythm section, often to the point (from listening to many of our members posted music) of obscuring the rhythm section altogether.

If you play with a real rhythm section, it doesn't matter how loud you play, the drummer and bass player usually have enough 'juice' left to rise up with you, and you never swamp them. You can always hear the drummer (usually TOO well!!), so you seldom lose the groove.

But do the same on an arranger, and your rhythm section just sits there, taking no notice of you..... It's all too easy to swamp them, and there goes your timing reference, and probably the groove.

The way to avoid this takes time, and a recorder of some sort (CD, cassette, built-in sequencer, etc.). Record yourself playing over your usual backing, then step back, clear your ears for a few minutes (listen to some jazz or classical, anything a bit different from what you just played), and then listen to what you just recorded, but here's the important thing - listen to it VERY QUIETLY.

It is much easier to hear when a lead voice is too loud at low volumes - at higher volumes you can still hear the other parts through the lead voice, but lower level listening quickly makes balance problems more noticeable. Now, does what you've recorded sound like the lead voice is a bit too loud, or does it 'sit' in the mix like a record? Does the lead ever obscure the drums a bit?

If so, go back to the registration (UPG or whatever your arranger calls the patch) and dial down the lead voice about 10%, and save (under a different name if you want to keep the original). Now record yourself doing the same song again, and (most importantly) try not to play harder to compensate for the drop in volume. Now listen again (after first clearing your ears for a few minutes) to the new recording, once again at VERY low levels. Is there ever a time when you lose the lead voice 'behind' the accompaniment? If not, dial down the lead voice another 10% and repeat.

Finally, there will come a level that you occasionally lose the lead voice. Now go back up 10% to the previous level and save. Now comes the hard part........ This new, 'correct' level may be 10-30% lower than you are used to, and you may want to hear yourself better, but don't give in to the temptation to play harder, or turn the lead back up. It IS the correct level..... You just have to get used to it.

But......... here's the best part of doing this - I guarantee you will be able to hear the rhythm section MUCH better now during your solos (and even better during the comping) and stand a much lesser chance of losing the groove. Your timing will improve, you will be listening more to the other things going on around you while you solo, and can interact more effectively. Plus your audience will appreciate a more even mix.

Over time, you will get used to hearing the correct amount of solo and comping volumes, and it becomes second nature, and timing problems will become more of a thing of the past.

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In fact, this repeated taping, listening and adjusting, then re-taping will help you cure a multitude of sins, from poor timing to bad registration choices, to having too much effects, to non-idiomatic instrument emulation. Just be patient, and focus on one problem at a time, and soon your playing and sound will improve dramatically.......